Handle Callback Errors

[thomasrichner-oviva]

In case a user aborts the login, we usually get a nice error in the callback.

/auth/callback?error=access_denied&error_reason=user_canceled_login&error_description=The+resource+owner+denied+the+request&state=...

Currently this is not handled well and a generic error is displayed to the user. The goal would be to give a bit more info to the user on what exactly went wrong.