Unit tests for authentication errors

[azmeuk]

Write some unit test, and document how to generate authentication errors.

https://openid.net/specs/openid-connect-core-1_0.html#AuthError
https://datatracker.ietf.org/doc/html/rfc6749#section-4.1.2.1

OAuth2 errors:

• invalid_request
• unauthorized_client
• access_denied
• unsupported_response_type
• invalid_scope
• server_error
• temporarily_unavailable

OIDC errors:

• interaction_required
• login_required
• account_selection_required
• consent_required
• invalid_request_uri
• invalid_request_object
• request_not_supported
• request_uri_not_supported
• registration_not_supported