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

src/Discord.Net.Rest/Entities/Messages/RestUserMessage.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
@@ -126,6 +126,8 @@ namespace Discord.Rest
 
         public async Task ModifyAsync(Action<MessageProperties> func, RequestOptions options = null)
         {
+            if (Author.Id != Discord.CurrentUser.Id)
+                throw new InvalidOperationException("Discord allows only the author of a message to change it.");
             var model = await MessageHelper.ModifyAsync(this, Discord, func, options).ConfigureAwait(false);
             Update(model);
         }

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;
@@ -122,7 +122,11 @@ namespace Discord.WebSocket
         }
 
         public Task ModifyAsync(Action<MessageProperties> func, RequestOptions options = null)
-            => MessageHelper.ModifyAsync(this, Discord, func, options);
+        {
+            if (Author.Id != Discord.CurrentUser.Id)
+                throw new InvalidOperationException("Discord allows only the author of a message to change it.");
+            return MessageHelper.ModifyAsync(this, Discord, func, options);
+        }
 
         public Task AddReactionAsync(IEmote emote, RequestOptions options = null)
             => MessageHelper.AddReactionAsync(this, emote, Discord, options);