Fix log line that was printing undefined value (#6278)

changelog.d/6278.bugfix

@@ -0,0 +1 @@
+Fix exception when remote servers attempt to join a room that they're not allowed to join.

synapse/handlers/federation.py

@@ -1250,7 +1250,7 @@ class FederationHandler(BaseHandler):
                 builder=builder
             )
         except AuthError as e:
-            logger.warn("Failed to create join %r because %s", event, e)
+            logger.warn("Failed to create join to %s because %s", room_id, e)
             raise e
 
         event_allowed = yield self.third_party_event_rules.check_event_allowed(