OIDC Auth: Implement response when user does not have role "user"

[SBlechmann]

Currently, when using OIDC Auth, there is no feedback when you try to login with a user that does not have the role "user". You are just redirected to the login page again without seeing any response.

Maybe there is something in the OIDC plugin or we just throw a custom message.

[SBlechmann]

When connected to the OICD provider, users must come with a (currently not necessarily verified) email address. Without email address, the login just ends up in a loop.