Fix exceptions from background database update for event labels. (#6407)

Add some exception handling here so that events whose json cannot be parsed are
ignored rather than getting us stuck in a loop.

Fixes #6404.

changelog.d/6407.bugfix

@@ -0,0 +1 @@
+Fix a bug which could cause the background database update hander for event labels to get stuck in a loop raising exceptions.

synapse/storage/data_stores/main/events_bg_updates.py

@@ -530,6 +530,7 @@ class EventsBackgroundUpdatesStore(BackgroundUpdateStore):
             nbrows = 0
             last_row_event_id = ""
             for (event_id, event_json_raw) in results:
+                try:
                     event_json = json.loads(event_json_raw)
 
                     self._simple_insert_many_txn(
@@ -548,6 +549,12 @@ class EventsBackgroundUpdatesStore(BackgroundUpdateStore):
                             if isinstance(label, str)
                         ],
                     )
+                except Exception as e:
+                    logger.warning(
+                        "Unable to load event %s (no labels will be imported): %s",
+                        event_id,
+                        e,
+                    )
 
                 nbrows += 1
                 last_row_event_id = event_id