correct gitlab authenticator when checking group membership of admins
correct gitlab authenticator when checking group membership of admins

We expect the API endpoint in question to return a 4xx return code
when the (admin) user is *not* in the group in question. We need
to check this explicitly, so 'http_client.fetch' should *not*
raise an exception if a non-2xx response is received.

jupyter/oauthenticator,yuvipanda/mwoauthenticator,enolfc/oauthenticator,maltevogl/oauthenticator,jupyterhub/oauthenticator,jupyter/oauthenticator,yuvipanda/mwoauthenticator,NickolausDS/oauthenticator,minrk/oauthenticator