Throw when attempting to modify a message not made by the current user (#992)

* Throw when attempting to modify a message not made by the current user

* Didn't realize the client is passed into the MessageHelper function

* Respond to feedback

src/Discord.Net.WebSocket/Entities/Messages/SocketUserMessage.cs

@@ -1,4 +1,4 @@
-using Discord.Rest;
+using Discord.Rest;
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;