Complete Preconditions implementation

src/Discord.Net.Commands/Results/ExecuteResult.cs

@@ -28,6 +28,8 @@ namespace Discord.Commands
             => new ExecuteResult(ex, CommandError.Exception, ex.Message);
         internal static ExecuteResult FromError(ParseResult result)
             => new ExecuteResult(null, result.Error, result.ErrorReason);
+        internal static ExecuteResult FromError(PreconditionResult result)
+            => new ExecuteResult(null, result.Error, result.ErrorReason);
         
         public override string ToString() => IsSuccess ? "Success" : $"{Error}: {ErrorReason}";
         private string DebuggerDisplay => IsSuccess ? "Success" : $"{Error}: {ErrorReason}";

src/Discord.Net.Commands/Results/PreconditionResult.cs

@@ -0,0 +1,27 @@
+using System.Diagnostics;
+
+namespace Discord.Commands
+{
+    [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
+    public struct PreconditionResult : IResult
+    {
+        public CommandError? Error { get; }
+        public string ErrorReason { get; }
+
+        public bool IsSuccess => !Error.HasValue;
+
+        private PreconditionResult(CommandError? error, string errorReason)
+        {
+            Error = error;
+            ErrorReason = errorReason;
+        }
+
+        internal static PreconditionResult FromSuccess()
+            => new PreconditionResult(null, null);
+        internal static PreconditionResult FromError(string reason)
+            => new PreconditionResult(CommandError.UnmetPrecondition, reason);
+
+        public override string ToString() => IsSuccess ? "Success" : $"{Error}: {ErrorReason}";
+        private string DebuggerDisplay => IsSuccess ? "Success" : $"{Error}: {ErrorReason}";
+    }
+}