500 Error page (/error) requires authentication

If a server error occurs that results in a 500 we are redirected to /error which in turn redirects to /login of we are not signed in.

This causes weird behavior when interacting with the OAuth API as we are not authenticated and the returned code for errors then becomes a 200 with /login after following all of the redirects.

Start by tracing the /error and /login redirect behavior for unauthenticated requests, then reproduce a server error through the OAuth API. Done means an unauthenticated 500 remains a 500 response instead of following redirects and returning a 200 login page. Check the existing work in linked pull request #973 before starting.

Written by the indexing model from the issue text.