Don't force a bad permission message

src/Discord.Net.Commands/CommandService.cs

@@ -9,8 +9,6 @@ namespace Discord.Commands
 	/// <summary> A Discord.Net client with extensions for handling common bot operations like text commands. </summary>
 	public sealed partial class CommandService : IService
     {
-		private const string DefaultPermissionError = "You do not have permission to access this command.";
-
         private readonly CommandServiceConfig _config;
 		private readonly CommandGroupBuilder _root;
 		private DiscordClient _client;
@@ -117,7 +115,7 @@ namespace Discord.Commands
 						string errorText;
 						if (!command.CanRun(eventArgs.User, eventArgs.Channel, out errorText))
 						{
-							RaiseCommandError(CommandErrorType.BadPermissions, eventArgs, new Exception(errorText ?? DefaultPermissionError));
+							RaiseCommandError(CommandErrorType.BadPermissions, eventArgs, errorText != null ? new Exception(errorText) : null);
 							return;
 						}
 
@@ -262,7 +260,7 @@ namespace Discord.Commands
 			StringBuilder output = new StringBuilder();
 			string error;
 			if (!command.CanRun(user, channel, out error))
-				output.AppendLine(error ?? DefaultPermissionError);
+				output.AppendLine(error ?? "You do not have permission to access this command.");
 			else
 				ShowCommandHelpInternal(command, user, channel, output);
             return (replyChannel ?? channel).SendMessage(output.ToString());