Access_Token endpoint returns Errors as XML only


The documentation on error messages says these would be formatted in the same format as the request.
The OAuth endpoints have no format for the request.

What I find strange though is that the request_token endpoint will return a plaintext error message, but the access_token endpoint returns XML.
Changing the extension of these URLs to .json won’t cause the error messages to be JSON encoded but stay XML.

This is especially wondering me as the whole 1.1 API does only support JSON.

Am I doing it wrong? Or is the XML output the only way to get the error message on that endpoint?