How to make an inputField required based on options from SelectList?

Question

The ask is: If the user selects 'Declined', in the "Envelope Status" Select list, the inputField "Declined Reason" should be mandatory. I have basically tried everything that is available in stackexchange; but couldn't figure out whats wrong in my code. Also, putting the inputField in an outputPanel, takes away its label. How to overcome that? Here is my Page:-

<apex:page standardController="Opportunity" extensions="NewDocuSignEnvelopeExtension" tabStyle="dsfs__DocuSign_Status__c">
<apex:form >
    <apex:sectionHeader subtitle="New DocuSign Envelope" title="Create Envelope"/>
    <apex:pageBlock title="New DocuSign Envelope" mode="edit">
        <apex:pageBlockButtons >
            <apex:commandButton value="Save" action="{!saveDocuSignRecord}"/>
            <apex:commandButton value="Cancel" action="{!cancel}" immediate="true"/>
        </apex:pageBlockButtons>
        <apex:pageBlockSection title="Information" collapsible="false" columns="2">
            <apex:outputText label="Sender" value="{!$User.FirstName} {!$User.LastName}"/>
            <apex:selectList id="envStatus" label="Envelope Status" value="{!envelopeStatus}" multiselect="false" size="1">
                <apex:selectOption itemValue="Sent" itemLabel="Sent"/>
                <apex:selectOption itemValue="Completed" itemLabel="Completed"/>
                <apex:selectOption itemValue="Declined" itemLabel="Declined"/>
                <apex:actionSupport event="onchange" action="{!checkRequired}" rerender="declinedReasonId"/>
            </apex:selectList>
            <apex:outputText label="Sender Email" value="{!$User.Email}"/>
            <apex:inputField value="{!status.Status_Date__c}" required="true"/>
            <apex:outputText label="Subject" value="Important eSignature Request from Bright House Networks"/>
            <apex:outputPanel id="declinedReasonId">
                <apex:inputField label="Declined Reason" value="{!status.dsfs__Declined_Reason__c}" required="{!isRequired}"/>
            </apex:outputPanel>
        </apex:pageBlockSection>
        <apex:pageBlockSection title="Related Objects" collapsible="false" columns="2">
            <apex:outputText label="Account" value="{!Opportunity.Account.Name}"/>
            <apex:outputText label="Opportunity" value="{!Opportunity.Name}"/>
        </apex:pageBlockSection>
    </apex:pageBlock>
</apex:form>

And My Extension code:-

public class NewDocuSignEnvelopeExtension {

public NewDocuSignEnvelopeExtension(ApexPages.StandardController controller) {

}

public NewDocuSignEnvelopeExtension() {

}

public dsfs__DocuSign_Status__c status{
    get{
        if(status == null)
            status = new dsfs__DocuSign_Status__c();
        return status;
    }
    set;}
public String envelopeStatus {get;set;}
public string declined {get;set;}
public boolean isRequired {get;set;}{isRequired=false;}

public PageReference saveDocuSignRecord(){
    String opptyId = ApexPages.currentPage().getParameters().get('Id');
    Opportunity oppty = new Opportunity();
    dsfs__DocuSign_Status__c ds = new dsfs__DocuSign_Status__c();
    Id recordTypeId = Schema.SObjectType.dsfs__DocuSign_Status__c.getRecordTypeInfosByName().get('Manual').getRecordTypeId();
    oppty=[select id, name, AccountId from Opportunity where id =: opptyId];
    ds.dsfs__Sender__c = UserInfo.getName();
    ds.dsfs__Sender_Email__c = UserInfo.getUserEmail();
    ds.dsfs__Subject__c = 'Important eSignature Request from Bright House Networks';
    ds.Lightning_Abhi__Envelope_status__c = envelopeStatus;
    ds.Lightning_Abhi__Status_Date__c = status.Status_Date__c;
    ds.dsfs__Declined_Reason__c = status.dsfs__Declined_Reason__c;
    ds.dsfs__Company__c = oppty.AccountId;
    ds.dsfs__Opportunity__c = oppty.Id;
    ds.RecordTypeId = recordTypeId;
    insert ds;
    PageReference dsPage = new PageReference('/' + ds.id);
    dsPage.setRedirect(true);
    return dsPage;
}

Public List<SelectOption> getItems(){
    List<SelectOption> options = new List<SelectOption>();
    options.add(new SelectOption('Sent','Sent'));
    options.add(new SelectOption('Completed','Completed'));
    options.add(new SelectOption('Declined','Declined'));
    return options;
}

public void checkRequired(){
    if(envelopeStatus=='Declined'){
        isRequired = true;
    }
}
}

Answer 1

Instead of calling into saveDocuSignRecord you would build a new method that would control any of the validation that would need to be done and then it would throw an error..

VF PAGE 
<apex:pageMessages id="feedback" escape="false" />

Controller
public void prepareDataForSave()
{
    Boolean result = true;
    String missingDataError = '';

    if(whatever you need to check)
    {
            missingDataError += '<br />  ' + 'Whatever you want your error to be';
            result = false;
    }

    if(result)
    {
        saveDocuSignRecord(result);
    }
    else
    {
        displayFeedback(ApexPages.Severity.Error, String.valueOf(missingDataError));
        missingDataError = '';
    } 
}

private void displayFeedback(ApexPages.Severity msgType, String message) 
{
    ApexPages.Message msg = new ApexPages.Message(msgType, message);
    ApexPages.addMessage(msg);
}

Answer 2

@EricSSH I haven't created another ticket as I have modified your code and now the page redirection is working just fine. The VF page snippet by you, is correct. I just had to modify the Apex controller code:-

public PageReference prepareDataForSave(){
Boolean result = true;
String missingDataError = '';

if(whatever you need to check)
{
    missingDataError += '<br />  ' + 'Whatever you want your error to be';
    result = false;
}

if(result)
{
    PageReference dsPage=saveDocuSignRecord();
    dsPage.setRedirect(true);
    return dsPage;
}
else
{
    displayFeedback(ApexPages.Severity.Error, String.valueOf(missingDataError));
    missingDataError = '';
    PageReference dsPage = new PageReference('/apex/NewDocuSignEnvelope?scontrolCaching=1&id='+opptyId);
    dsPage.setRedirect(false);
    return dsPage;
}
}

private void displayFeedback(ApexPages.Severity msgType, String message){
ApexPages.Message msg = new ApexPages.Message(msgType, message);
ApexPages.addMessage(msg);
}