Form errors persist on resubmission

[Dave Shoreman]

If there are errors when you save a site, errors for all fields are correctly displayed including the top "Could not save Site!" alert.

However when you fix the error (for field validation) or resubmit the form (for general errors when saving), the errors don't get cleared even if the latter submission is successful.

We could add frontend validation that deals with all the per-field errors, but we'd still need to deal with the top alert box in case of server errors causing the save to fail. It's probably easier to just reset the errors var in Vue before the ajax call is fired...