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.
Migrated from D6546 (view on Phabricator)