Handle (other) users that exist in Alma but don't in LDAP

[lazka]

In case there exist users in Alma that doesn't in LDAP (because the user got removed) we should communicate that to the frontend and don't fail for operations where they are optional:

Assume there is a user NONEXIST that is missing in LDAP:

• Borrow book: If NONEXIST has already borrowed it we shouldn't fail with an alma-id error, instead with "Existing loan was found!".

• Return book: If it gets returned for NONEXIST then maybe we can ignore the error? And just show "Borrower: Unknown"

• Show current loan list: Show the loans by NONEXIST with "Unknown" instead of the email button.

In case we return a "Person" from the API things get tricky. Not sure what to do there.. maybe just return null and let the frontend handle it..