Throw an exception if session can't be destroyed

plugins/cas_auth_unl/start.php

@@ -128,7 +128,9 @@ class UnlCAS {
                 $data = $item->get();
 
                 $handler = new Elgg\Http\DatabaseSessionHandler(_elgg_services()->db);
-                $handler->destroy($data['session_id']);
+                if (!$handler->destroy($data['session_id'])) {
+                    throw new \Exception('unable to destroy session on single sign out');
+                }
                 
                 //Remove the cached item
                 $item->clear();