When logout-initiating SP has no known SLO endpoint, fallback to portal
In some federations, SLO endpoints are not published, yet SP are still able to initiate logouts.
This used to cause an error on the portal, this commit changes the error into a warning in logs. The user remains on the portal and sees a normal logout message.
This should eventually fix #1668 (closed)
Work in progress until #1671 (closed) is solved, because this needs to be tested with multiple SPs too.
Edited by Maxime Besson