New: Add result to commands to report commands that did not complete successfully

(cherry picked from commit 103ce3def4636ef891e72bd687ef8f46b5125233)

src/NzbDrone.Core/Messaging/Commands/CommandModel.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using NzbDrone.Common.Messaging;
 using NzbDrone.Core.Datastore;
 
@@ -10,6 +10,7 @@ namespace NzbDrone.Core.Messaging.Commands
         public Command Body { get; set; }
         public CommandPriority Priority { get; set; }
         public CommandStatus Status { get; set; }
+        public CommandResult Result { get; set; }
         public DateTime QueuedAt { get; set; }
         public DateTime? StartedAt { get; set; }
         public DateTime? EndedAt { get; set; }