Improve user flow when Group SAML linking fails

What

When SAML linking fails we should redirect users to the page where they can unlink their account, or otherwise improve the user flow so they are not left stranded/confused.

Why

The messages User has already been taken and Extern uid has already been taken are confusing and will be improved in #33713 (closed).

Even after improving them we should make it easier for users to unlink existing broken links.