Add assertion message to better debug

synapse/storage/databases/main/events_bg_updates.py

@@ -2247,6 +2247,10 @@ class EventsBackgroundUpdatesStore(StreamWorkerStore, StateDeltasStore, SQLBaseS
                     assert (
                         last_to_insert_join_membership_info.membership_event_stream_ordering
                         <= membership_event_stream_ordering
+                    ), (
+                        f"Expected to be iterating in ascending order as we process membership snapshots for room {room_id}"
+                        + " but our last join membership was at {last_to_insert_join_membership_info.membership_event_stream_ordering}"
+                        + "and our current membership is at {membership_event_stream_ordering}"
                     )
 
                     # Check if the current state has been updated since the last snapshot we inserted.