prevent custom google form to redirect on error

hi all, I have successfully set up a redirected form that works on submission using by using the following

<script type="text/javascript">var submitted=false;</script>

    <iframe name="hidden_iframe" id="hidden_iframe" style="display:none;"     

onload="if(submitted) {window.location='whateveryourredirectis.html';}"></iframe>

      <form action="YOUR GOOGLE FORM" method="post" target="hidden_iframe" 

onsubmit="submitted=true;">

BUT, I have a couple of requirements on my form (ie. maximum characters etc.) that if they return an error, the form gets redirected to the original form saying ""Looks like you have a question or two that still needs attention."

Does anyone know if I can still have my submission redirect AND also have a redirect for errors (or better still to list the error on my custom form)?

thanks!