Display a user-friendly warning on a 401 response to /.well-known

Catch a 401 Unauthorized response to `GET https://localhost:8443/.well-known/openid-configuration` during client registration, and warn the user in the console, something like "Cannot register client, server is not in OIDC auth mode".

(Inspired by https://github.com/solid/oidc-auth-manager/issues/6#issuecomment-296525328)