I'm not comfortable at all with setting arbitrarily the responseText as error message. I think, in order to use the ajax call response as the message, that user must format a minimum its response.

I think we should "force" the user to format his response in json first.

Then, "force" him to format it this way (@wisembly format :p) `{error:{message: "error message"}}` or `{message: "error message"}` (@github format).