api/graph: Fix error responses for invalid request Accept header

When setting an unsupported value for the request Accept header when
querying the /api/1/graph/ endpoint, an error was raised as the Django
DRF response does not have the accepted_media_type attribute set in
that case, leading to a 500 HTTP status code.

So ensure a proper JSON error response will be generated with the
406 HTTP status code when submitting such requests.