Add error handling

[ftsell]

Currently, any login errors (such as aborted logins or token exchange errors) are completely ignored. This is obviously not good and should be improved.

At least the following error handlers should be implemented:

• The provided OpenidConfig can be invalid which results in getServiceConfiguration not succeeding
• Token renewal in renewTokens may fail if the refresh token is expired or the user has revoked the session (logged out)
• The authorization flow may fail or get aborted which results in doAuthorizationCallback not succeeding because no code is available as query parameter. Instead, the authorization notifier will be called with a non-null error that needs to be handled.