fix for issue #430

Firstly, apologies if I've done something wrong here, my first effort at contributing a fix. I *think* I've committed against an issue, though I may have done it completely wrong!

This covers the case where the the form has failed server-side validation and so the form is sent back to the user with server-generated error labels. Currently the validation plugin will not update these labels if new user input causes client-side validation to fail. This commit fixes that.

I made the assumption that any existing label which already has the errorClass assigned is safe to update with a new message, which i believe is a safe assumption (why would any label other than a validation error label, already have the errorClass assigned?)

Thanks,
Nick