Change exceptions which should be returned to the user to HubException type

See
https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.signalr.hubexception?view=aspnetcore-5.0.

osu.Game/Online/RealtimeMultiplayer/AlreadyInRoomException.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 AlreadyInRoomException : Exception
+    [Serializable]
+    public class AlreadyInRoomException : HubException
     {
         public AlreadyInRoomException()
             : base("This user is already in a multiplayer room.")
         {
         }
+
+        protected AlreadyInRoomException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
     }
 }

osu.Game/Online/RealtimeMultiplayer/InvalidStateChangeException.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 InvalidStateChangeException : Exception
+    [Serializable]
+    public class InvalidStateChangeException : HubException
     {
         public InvalidStateChangeException(MultiplayerUserState oldState, MultiplayerUserState newState)
             : base($"Cannot change from {oldState} to {newState}")
         {
         }
+
+        protected InvalidStateChangeException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
     }
 }

osu.Game/Online/RealtimeMultiplayer/NotHostException.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 NotHostException : Exception
+    [Serializable]
+    public class NotHostException : HubException
     {
         public NotHostException()
             : base("User is attempting to perform a host level operation while not the host")
         {
         }
+
+        protected NotHostException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
     }
 }

osu.Game/Online/RealtimeMultiplayer/NotJoinedRoomException.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 NotJoinedRoomException : Exception
+    [Serializable]
+    public class NotJoinedRoomException : HubException
     {
         public NotJoinedRoomException()
             : base("This user has not yet joined a multiplayer room.")
         {
         }
+
+        protected NotJoinedRoomException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
     }
 }