Improve Group SAML authentication failed messages

What

Create improved error messages for the following:

• Extern uid has already been taken: actually caused by trying to log into multiple users with one identity.
• User has already been taken: actually caused by mismatched NameID
• Both of the above: actually relates to NameID capitalization
• Ensure there is a sensible error message when NameID is missing

Update documentation for these under https://docs.gitlab.com/ee/user/group/saml_sso/#message-saml-authentication-failed-extern-uid-has-already-been-taken

Why

These messages are currently confusing and don't help diagnose NameID misconfiguration. A first guess with the current messages might be the opposite of what is actually occurring.

Relates to #33464 (closed)