Handle incomplete saml configuration more gracefully

[kesselb]

Steps to reproduce

1. Enable user_saml
2. Use built-in SAML authentication
3. Look around, do not configure saml properly
4. Go to sleep
5. Visit Nextcloud (the session expired, and I was redirected to the login page)
6. Greeted by an error message

Expected behaviour

I think it would be possible to handle such an incomplete but enabled saml configuration more gracefully.

Actual behaviour

An incomplete saml configuration leads to an error message on the login page.

Incomplete saml configuration => Layer 8 problem, although we could handle it better.

Server configuration

PHP version: 8.3

Nextcloud version: master/30 (see Nextcloud admin page)

Where did you install Nextcloud from: git