According to http://api.jquery.com/jQuery.ajax/ the fail callback should be getting:
jqXHR, textStatus, errorThrown
same as error, but error is deprecated:
Deprecation Notice: The jqXHR.success(), jqXHR.error(), and jqXHR.complete() callbacks will be deprecated in jQuery 1.8. To prepare your code for their eventual removal, use jqXHR.done(), jqXHR.fail(), and jqXHR.always() instead.