Shift focus on to the error field on a page load in Visualforce page

Question

The focus stays on the first field on a page load,which is the one highlighted in the image "Who prompted you.."

But I would like to change the default focus ,whenever there is an error(system or a validation)

For example:
When there is an error on page save,like as below, the focus should shift to the error field,like in this case Business name.

The field I am referring is "existingbusiness" which is a lookup field. When user types in a text and presses enter,the lookup field errors out as it finds a multiple match.This is achieved by the actionfunction which calls a donotSubmit function. The objective is to change the focus to this error field and shift the the default focus from the first field on the form.

VF page:

<apex:page extensions="PNTReferralInfoController" standardController="Referral__c" tabStyle="Account" title="PNT Referral Information" id="thePage" >
<script type='text/javascript'>

function setFocusOnLoad()
{
    document.getElementById('{!$Component.theForm.block.sec.reason}').focus();

}}
window.onload=setFocusOnLoad;
</script>
<script type='text/javascript'>
function noenter(ev)  {
if (window.event && window.event.keyCode == 13 || ev.which == 13) {
    donotSubmit();
    return false;
} else {
    return true;
}
}
</script>

<apex:form id="theForm">
<apex:pageBlock title="Referral Information" id="block" mode="edit">
    <apex:pageMessages id="errorMessages"/>
    <apex:pageBlockSection columns="1" collapsible="false" id="sec">



        <apex:pageblock id="ExistingBusinessAccountInformation" mode="mainDetail" >
            <apex:pageBlockSection columns="1" collapsible="false" rendered="{!showexistingbusinessdetails}">

                <h1>Option 1: Select EXISTING Business</h1>
                <apex:pageBlockSectionItem >
                    <apex:outputLabel value="Business Name" for="existingBusiness" />
                    <apex:actionRegion >
                        <apex:inputField value="{!referral.Referrer_Organisation__c}" required="false" id="existingBusiness" onkeypress="return noenter(event)" >
                            <apex:actionSupport event="onchange"  action="{!existingBusiness}" reRender="Newbusiness,message2"  />
                        </apex:inputField> 
                    </apex:actionRegion>
                </apex:pageblocksectionitem>
            </apex:pageBlockSection>
        </apex:pageBlock>


<apex:pageBlockButtons location="bottom" > 
<apex:actionFunction name="donotSubmit" action="{!donotSubmit}" immediate="true"/>
<apex:commandButton id="previous" value="Previous" action="{!back}" immediate="true" />
<apex:commandButton id="savepage" value="Save" action="{!saveReferralInformation}" />
<apex:commandButton id="resetBtn" value="Cancel/Start Again" immediate="true" action="{!reset}"/>
</apex:pageBlockButtons>
</apex:pageBlock>
</apex:form>
</apex:page>

Apex controller:

public class PNTReferralInfoController {
    public ApexPages.StandardController standardController {get;set;} 
    public Referral__c referral {get;set;}
    public String personId {get;set;}
    public String contactId {get;set;}
    public Account person {get;set;}   
    public Account client {get;set;}

    public PNTReferralInfoController( ApexPages.StandardController controller){ 
        standardController = controller;
        referral = (Referral__c)standardController.getRecord();
        personId = ApexPages.currentPage().getParameters().get('personId');
        person = this.getPerson(personId);        
        if(person != null){referral.Client__c = person.Id;}
        client = [SELECT id,Lead_Source__pc,Eye_Condition__pc,Reason_for_contact__pc,Other_Vision_Condition_Information__pc  from Account where id=:personId];

        referrer = new Account();
        contactRecordtype = [SELECT Id FROM RecordType WHERE SobjectType = 'Account' AND DeveloperName = 'Contact' limit 1];
        referrer.RecordTypeId = contactRecordtype.Id;   
        tempContact = new Contact();
        newBusiness = new Account();
        newBusinessAccount = new Account();
        newRelationship = new Relationship__c();
        linkBusinessAccounttoReferrer = new AccountContactRelation();
        removelinkedbusinessdetails = true;
        showexistingbusinessdetails = false;
        shownewbusinessdetails = false;
        bid = false;
        matchText = NULL;
        Search_results = 1;
        Search_results_ba_multi =-1;
        Search_results_ba_blank =-1;

    }


    public PageReference saveReferralInformation(){
        try{
            person.Reason_for_contact__pc = client.Reason_for_contact__pc;
            person.Last_PNT_Run_Date__pc = Datetime.now();

            upsert person;
        }
        catch(DmlException ex){ApexPages.addMessages(ex);}

        PNTClientInfoController.savePNTFormDataSnapShot(person.id,referral.Id);
        return new PageReference('/'+ person.Id);
    }

    public PageReference reset(){
        person = null;
        PageReference pageRef = Page.PNT_Client;
        pageRef.setRedirect(true);
        return pageRef;
    }

    public PageReference back(){
        PageReference pageref = new PageReference('/apex/pnt_client?id='+ personId);
        pageref.setRedirect(true);
        return pageref;

    } 

    public PageReference donotSubmit()
    {

        showexistingbusinessdetails = true;
        return null;
    }

}

Answer 1

use oncolplete() on actionFunction to reset focus by calling the javascript method