Support custom error callbacks

[dweinand]

If a backend returns a status code other than 200, then the response body is split on newlines and dumped into the form. This is problematic if the backend is returning a json data structure, since it will be dumped into the page unparsed.

See: http://jsfiddle.net/xBB5x/1045/

Right now the only options are to change the backend to return plain text, or to return a 200 and then use a custom success handler.

Altering the backend for a single plugin, is not a particularly attractive choice. It would be better if there was support for a custom error callback that worked just like the custom success callback, but was used when the backend returned an error code.

[vitalets]

Agree, good case.
will try to implement in the nearest release. thx!