エラー処理の後にポップアップメッセージ

ErrorPopupController.cls

public with sharing class ErrorPopupController {
	
    public Contact contact {get; set;}
    public String errorMsg {get; set;}
	
    public ErrorPopupController() {
        this.contact = new Contact();
        this.errorMsg = 'bat';
    }
	
    public PageReference doClick() {
        if (String.isEmpty(this.contact.AccountId) == true) {
            this.contact.AccountId.addError(this.errorMsg);
            return null;
        }
        return new PageReference('/' + this.contact.AccountId);
    }
}

ErrorPopupPage.page

<apex:page controller="ErrorPopupController" title="ErrorPopupPage" showheader="true" sidebar="false" tabStyle="Contact" id="page">
    <apex:form id="form">
        <apex:pageBlock >
            <apex:pageBlockButtons location="bottom">
                <apex:commandButton value=" Click!! " title=" Click!! " action="{!doClick}"
                    onComplete="return errorPopup();" reRender="form" />
            </apex:pageBlockButtons>
            <apex:pageBlockSection >
                <apex:inputField value="{!contact.AccountId}" />
            </apex:pageBlockSection>
        </apex:pageBlock>
        <script>
            function errorPopup() {
                alert('{!JSENCODE(errorMsg)}');
                return false;
            }
        </script>
    </apex:form>
</apex:page>