SYN-154: Better error messages when joining an unknown room by ID.

The simple fix doesn't work here because room creation also involves
unknown room IDs. The check relies on the presence of m.room.create for
rooms being created, whereas bogus room IDs have no state events at all.

synapse/handlers/room.py

@@ -423,12 +423,13 @@ class RoomMemberHandler(BaseHandler):
 
         is_host_in_room = yield self.auth.check_host_in_room(
             event.room_id,
-            self.hs.hostname
+            self.hs.hostname,
+            context=context
         )
 
         if is_host_in_room:
             should_do_dance = False
-        elif room_host:
+        elif room_host:  # TODO: Shouldn't this be remote_room_host?
             should_do_dance = True
         else:
             # TODO(markjh): get prev_state from snapshot
@@ -442,7 +443,8 @@ class RoomMemberHandler(BaseHandler):
                 should_do_dance = not self.hs.is_mine(inviter)
                 room_host = inviter.domain
             else:
-                should_do_dance = False
+                # return the same error as join_room_alias does
+                raise SynapseError(404, "No known servers")
 
         if should_do_dance:
             handler = self.hs.get_handlers().federation_handler