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.
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