Comment 1 for bug 1311573

There's neither an XML interface nor a JSON interface to this call.

The only supported content type on this call is "application/x-pem-file". The content of of the error->message attribute in your example response should be the entire response body itself, regardless of whether the client demands an application/xml response or not.

The XML translation layer is also deprecated as of Icehouse, due to the high cost of maintenance and a lack of interest in the community to support it (hence the Won't Fix).