Fix exception when failing to get remote room list (#10414)

Author: erikjohnston

changelog.d/10414.bugfix

@@ -0,0 +1 @@
+Fix a number of logged errors caused by remote servers being down.

synapse/handlers/room_list.py

@@ -383,7 +383,11 @@ async def get_remote_public_room_list(
                 ):
                     logger.debug("Falling back to locally-filtered /publicRooms")
                 else:
-                    raise  # Not an error that should trigger a fallback.
+                    # Not an error that should trigger a fallback.
+                    raise SynapseError(502, "Failed to fetch room list")
+            except RequestSendFailed:
+                # Not an error that should trigger a fallback.
+                raise SynapseError(502, "Failed to fetch room list")
 
             # if we reach this point, then we fall back to the situation where
             # we currently don't support searching across federation, so we have