Returning JSON from `login_user` for third party auth failures makes
the response more consistent with all other `login_user` responses.
The only calls to `login_user` with this failure are processed by
`shim_student_view` which will in-turn remove this JSON. This improves
the `login_user` response in advance of switching the logistration page
to use `login_user` without `shim_student_view`.
ARCH-1253
ebcff3fb4b