Detect and show AJAX errors on FormXHR submission handling.

Detect and show AJAX errors on FormXHR submission handling.

Previously we threw away errore reported by the actual AJAX submission and only used whatever error the browser reported to us; this didn't help if we reached the server ok but had a problem there.
We now extract the #error if given and use that in the alert().
Also should now show 'Unknown error' instead of a crappy generic 'something died in jQuery' when we get a 200 response but bogus return data.