Add error message in chat when attempting to use commands

osu.Game/Online/Chat/ErrorMessage.cs

@@ -0,0 +1,25 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System;
+using osu.Game.Users;
+
+namespace osu.Game.Online.Chat
+{
+    public class ErrorMessage : Message
+    {
+        private static int errorId = -1;
+
+        public ErrorMessage(string message) : base(errorId--)
+        {
+            Timestamp = DateTime.Now;
+            Content = message;
+
+            Sender = new User
+            {
+                Username = @"system",
+                Colour = @"ff0000",
+            };
+        }
+    }
+}