Relay Insights config format errors to the frontend

The following discussion from !17589 (merged) should be addressed:

• @himkp started a discussion: (+2 comments)

  Shouldn't this be fixed on the backend and it should not return a 200 status when clearly there was an error? If there was a server error, it should send back status 5xx. If there was an issue in the request, it should send 4xx. This particular case feels like a 400 error to me.