Added custom error to AddCheck(lambda)

src/Discord.Net.Commands/CommandBuilder.cs

@@ -75,9 +75,9 @@ namespace Discord.Commands
 			_checks.Add(check);
 			return this;
 		}
-		public CommandBuilder AddCheck(Func<Command, User, Channel, bool> checkFunc)
+		public CommandBuilder AddCheck(Func<Command, User, Channel, bool> checkFunc, string errorMsg = null)
 		{
-			_checks.Add(new GenericPermissionChecker(checkFunc));
+			_checks.Add(new GenericPermissionChecker(checkFunc, errorMsg));
 			return this;
 		}
 
@@ -140,9 +140,9 @@ namespace Discord.Commands
 		{
 			_checks.Add(checker);
 		}
-		public void AddCheck(Func<Command, User, Channel, bool> checkFunc)
+		public void AddCheck(Func<Command, User, Channel, bool> checkFunc, string errorMsg = null)
 		{
-			_checks.Add(new GenericPermissionChecker(checkFunc));
+			_checks.Add(new GenericPermissionChecker(checkFunc, errorMsg));
 		}
 
 		public CommandGroupBuilder CreateGroup(string cmd, Action<CommandGroupBuilder> config = null)

src/Discord.Net.Commands/Permissions/GenericPermissionChecker.cs

@@ -5,15 +5,17 @@ namespace Discord.Commands.Permissions
     internal class GenericPermissionChecker : IPermissionChecker
     {
 		private readonly Func<Command, User, Channel, bool> _checkFunc;
+        private readonly string _error;
 
-		public GenericPermissionChecker(Func<Command, User, Channel, bool> checkFunc)
+		public GenericPermissionChecker(Func<Command, User, Channel, bool> checkFunc, string error = null)
 		{
 			_checkFunc = checkFunc;
+            _error = error;
         }
 
 		public bool CanRun(Command command, User user, Channel channel, out string error)
 		{
-			error = null; //Use default error text.
+			error = _error;
 			return _checkFunc(command, user, channel);
         }
 	}