Fix failure to fetch batches of PDUs (#5342)

FederationClient.get_pdu is called in a loop to fetch a batch of PDUs. A
failure to fetch one should not result in a failure of the whole batch. Add the
missing `continue`.

changelog.d/5342.bugfix

@@ -0,0 +1 @@
+Fix failure when fetching batches of events during backfill, etc.

synapse/federation/federation_client.py

@@ -279,6 +279,7 @@ class FederationClient(FederationBase):
                     "Failed to get PDU %s from %s because %s",
                     event_id, destination, e,
                 )
+                continue
             except NotRetryingDestination as e:
                 logger.info(str(e))
                 continue