Fix exceptions when fetching events from a down host.

changelog.d/7622.bugfix

@@ -0,0 +1 @@
+Fix exceptions when fetching events from a remote host fails.

synapse/handlers/federation.py

@@ -501,7 +501,7 @@ async def _get_missing_events_for_pdu(self, origin, pdu, prevs, min_depth):
                 min_depth=min_depth,
                 timeout=60000,
             )
-        except RequestSendFailed as e:
+        except (RequestSendFailed, HttpResponseException, NotRetryingDestination) as e:
             # We failed to get the missing events, but since we need to handle
             # the case of `get_missing_events` not returning the necessary
             # events anyway, it is safe to simply log the error and continue.