Correctly handle failing to parse event dict from DB

Now that we do a bit more validadtion of events, it's possible that an
event persisted in the database may now not pass validation. This
shouldn't happen, but let's handle it correctly by logging and returning
that we couldn't find the event.

This is the same as what we do if we can't parse the JSON.

synapse/storage/databases/main/events_worker.py

@@ -1516,12 +1516,17 @@ class EventsWorkerStore(SQLBaseStore):
                     )
                     continue
 
-            original_ev = make_event_from_dict(
-                event_dict=d,
-                room_version=room_version,
-                internal_metadata_dict=internal_metadata,
-                rejected_reason=rejected_reason,
-            )
+            try:
+                original_ev = make_event_from_dict(
+                    event_dict=d,
+                    room_version=room_version,
+                    internal_metadata_dict=internal_metadata,
+                    rejected_reason=rejected_reason,
+                )
+            except ValueError:
+                logger.error("Unable to parse event from database: %s", event_id)
+                continue
+
             original_ev.internal_metadata.stream_ordering = row.stream_ordering
             original_ev.internal_metadata.instance_name = row.instance_name
             original_ev.internal_metadata.outlier = row.outlier