Only realistic way I can think of is to get each and every possible error and then do tags in the templates like
<script>
<%if Error = "Your username is already taken%>
document.forms.signupform.Username.focus;
alert("Your Username is already taken");
<%endif>
</script>
<script>
<%if Error = "Your username is already taken%>
document.forms.signupform.Username.focus;
alert("Your Username is already taken");
<%endif>
</script>
