SAML errors should be shown in the UI rather than a generic error

https://gitlab.example.com/users/auth/saml/omniauth_error?error=Email+can%27t+be+blank%2C+Notification+email+can%27t+be+blank%2C+and+Notification+email+is+invalid

This error occurs if the SAML response does not contain the email claim (of a custom claim name, if so configured).

On the page itself there is a 422 error that reads "The change you requested was rejected. Make sure you have access to the thing you tried to change. Please contact your GitLab administrator if you think this is a mistake."

Since the error is available we should display it in a nice and useful way.