Exit match sub screen when a room goes away

Closes #13847.

I think we can probably get some test coverage of this if required, but
needs a bit of thought (basically an error needs to be thrown during the
multiplayer client portion of the join procedure, after `CurrentRoom` is
non-null but before the join completes).

Manual testing on password branch (#13861) is possible since it
currently errors due to missing method on the live/dev servers.

- Create game, which will fail with `MethodNotExists`.
- Note the fields on the settings screen are emptied.
- Fill fields again and press create game (crash).

osu.Game/Screens/OnlinePlay/Multiplayer/MultiplayerMatchSubScreen.cs

@@ -48,6 +48,9 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
         [Resolved]
         private OngoingOperationTracker ongoingOperationTracker { get; set; }
 
+        [Resolved]
+        private Bindable<Room> currentRoom { get; set; }
+
         private MultiplayerMatchSettingsOverlay settingsOverlay;
 
         private readonly IBindable<bool> isConnected = new Bindable<bool>();
@@ -273,6 +276,16 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
                 if (!connected.NewValue)
                     Schedule(this.Exit);
             }, true);
+
+            currentRoom.BindValueChanged(room =>
+            {
+                if (room.NewValue == null)
+                {
+                    // the room has gone away.
+                    // this could mean something happened during the join process, or an external connection issue occurred.
+                    Schedule(this.Exit);
+                }
+            }, true);
         }
 
         protected override void UpdateMods()