Catch ValueError when trying to retrieve existing credentials
    
    get_credentials() was changed to raise ValueError instead of
    gssapi.exceptions.GSSError as part of the sweeper to clean up
    expired credentials caches.
    
    For WebUI users, this will prevent a 500 error if their
    associated credentials cache is expired or missing.
    
    https://pagure.io/freeipa/issue/8873
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>