Also make InvalidStateException serializable

osu.Game/Online/RealtimeMultiplayer/InvalidStateException.cs

@@ -2,14 +2,22 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Runtime.Serialization;
+using Microsoft.AspNetCore.SignalR;
 
 namespace osu.Game.Online.RealtimeMultiplayer
 {
-    public class InvalidStateException : Exception
+    [Serializable]
+    public class InvalidStateException : HubException
     {
         public InvalidStateException(string message)
             : base(message)
         {
         }
+
+        protected InvalidStateException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
     }
 }