Is it possible to show standard and custom error message at the same time?

Question

I want to know that is there a method to show the standard error message and custom error message at the same time? Also, in my script, there are several custom error messages. Is it possible to show them at the same time? Recently, the error message can only be showed one by one, due to the apex. Is there any way to fix it?

Here is the background for my page. It is a form to fill in information, and also attachment. Some of the essential information and attachment are required to fill in. However, if I input none of the field and attachment, it will only show the standard error message first. Only when I input all the field with standard error message, and the attachment field is blank, the custom error message finally shows.

Here is my code for your reference:

public class extattachfile {
    Public attachment objAttachment{get;set;}
    Public attachment objAttachment2{get; set;}
    Public attachment objAttachment3{get; set;}
    Public attachment objAttachmentt{get; set;}
    Public attachment objAttachments{get; set;}
    Public Artist__c artist{get; set;}
    Public extattachfile(apexpages.standardcontroller stdCon) {
        objAttachment = new Attachment();
        objAttachment2 = new Attachment();
        objAttachment3 = new Attachment();
        objAttachmentt = new Attachment();
        objAttachments = new Attachment();
        artist= new Artist__c ();
    }
    public PageReference save() {
        Boolean checkAttachment = false;
        Boolean isValidUrl = true;
        if(artist.Id == null){
            if(artist.Website__c != null){
                Pattern emailPattern = Pattern.compile('^((http|https)://)??(www[.])??([a-zA-Z0-9]|-)+?([.][a-zA-Z0-9(-|/|=|?)??]+?)+?$');
                Boolean isMatch = emailPattern.matcher(artist.Website__c).matches();
                if(!isMatch){
                    objAttachment.Body = null;
objAttachment2.Body = null;
objAttachment3.Body = null;
objAttachmentt.Body = null;
objAttachments.Body = null;
                    ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR, 'Please provide a valid Website URL.'));
                    isValidUrl = false;
                }
            }
            if(artist.Facebook__c != null){
                Pattern emailPattern = Pattern.compile('^((http|https)://)??(www[.])??([a-zA-Z0-9]|-)+?([.][a-zA-Z0-9(-|/|=|?)??]+?)+?$');
                Boolean isMatch = emailPattern.matcher(artist.Facebook__c).matches();
                if(!isMatch){
                    objAttachment.Body = null;
objAttachment2.Body = null;
objAttachment3.Body = null;
objAttachmentt.Body = null;
objAttachments.Body = null;
                    ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR, 'Please provide a valid Facebook URL.'));
                    isValidUrl = false;
                }
            }
            if(artist.SoundCloud__c != null){
                Pattern emailPattern = Pattern.compile('^((http|https)://)??(www[.])??([a-zA-Z0-9]|-)+?([.][a-zA-Z0-9(-|/|=|?)??]+?)+?$');
                Boolean isMatch = emailPattern.matcher(artist.SoundCloud__c).matches();
                if(!isMatch){
                    objAttachment.Body = null;
objAttachment2.Body = null;
objAttachment3.Body = null;
objAttachmentt.Body = null;
objAttachments.Body = null;
                    ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR, 'Please provide a valid Soundcloud URL.'));
                    isValidUrl = false;
                }
            }
            if(artist.YouTube__c != null){
                Pattern emailPattern = Pattern.compile('^((http|https)://)??(www[.])??([a-zA-Z0-9]|-)+?([.][a-zA-Z0-9(-|/|=|?)??]+?)+?$');
                Boolean isMatch = emailPattern.matcher(artist.YouTube__c).matches();
                if(!isMatch){
                    objAttachment.Body = null;
objAttachment2.Body = null;
objAttachment3.Body = null;
objAttachmentt.Body = null;
objAttachments.Body = null;
                    ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR, 'Please provide a valid Youtube URL.'));
                    isValidUrl = false;
                }
            }
            if(isValidUrl){
                insert artist;
            }else{
                return null;
            }
        }
        List<Attachment> attachmentList = new List<Attachment>();
        if(objAttachment.Body != null){
            objAttachment.ParentId = artist.id;
            List<String> splitStrList = objAttachment.Name.Split('\\.');
            String fileExtension = splitStrList[1];
            objAttachment.Name = artist.Id+'_'+artist.Artist_Group_Name_English__c+'_'+String.valueOf(system.today())+'_Photo1'+'.' + fileExtension;
            attachmentList.add(objAttachment);
            checkAttachment = true;
        }
        if(objAttachment2.Body != null){
            objAttachment2.ParentId = artist.id;
            List<String> splitStrList = objAttachment2.Name.Split('\\.');
            String fileExtension = splitStrList[1];
            objAttachment2.Name = artist.Id+'_'+artist.Artist_Group_Name_English__c+'_'+String.valueOf(system.today())+'_Photo2'+'.' + fileExtension;
            attachmentList.add(objAttachment2);
            checkAttachment = true;
        }
        if(objAttachment3.Body != null){
            objAttachment3.ParentId = artist.id;
            List<String> splitStrList = objAttachment3.Name.Split('\\.');
            String fileExtension = splitStrList[1];
            objAttachment3.Name = artist.Id+'_'+artist.Artist_Group_Name_English__c+'_'+String.valueOf(system.today())+'_Photo3'+'.' + fileExtension;
            attachmentList.add(objAttachment3);
            checkAttachment = true;
        }          
        List<Attachment> attachmentListother = new List<Attachment>();
        if(objAttachmentt.Body != null){
            objAttachmentt.ParentId = artist.id;
            List<String> splitStrList = objAttachmentt.Name.Split('\\.');
            String fileExtension = splitStrList[1];
            objAttachmentt.Name = artist.Id+'_'+artist.Artist_Group_Name_English__c+'_'+String.valueOf(system.today())+'_Technical'+'.' + fileExtension;
            attachmentList.add(objAttachmentt);
        }
        if(objAttachments.Body != null){
            objAttachments.ParentId = artist.id;
            List<String> splitStrList = objAttachments.Name.Split('\\.');
            String fileExtension = splitStrList[1];
            objAttachments.Name = artist.Id+'_'+artist.Artist_Group_Name_English__c+'_'+String.valueOf(system.today())+'_Stage'+'.' + fileExtension;
            attachmentList.add(objAttachments);
        }
        Insert attachmentListother;
        if(attachmentList.size() > 0 && checkAttachment){
            insert attachmentList;
            attachmentList = null;
            // if successfully inserted new contact, then displays the thank you page.
            return Page.ack;
        }else{
            ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR, 'Please attach at least one photo attachment.'));
            return null;
        }             
    }
}

Answer 1

No. Much like standard validation rules, they calculated during a different phase of the page lifecycle. Your custom code will never run if standard validation fails. There's really no way to change this. All of the messages must be custom.