Getting model by invalid ID throws a more specific exception.

NzbDrone.Core/Datastore/ModelNotFoundException.cs

@@ -0,0 +1,14 @@
+using System;
+using NzbDrone.Common.Exceptions;
+
+namespace NzbDrone.Core.Datastore
+{
+    public class ModelNotFoundException : NzbDroneException
+    {
+        public ModelNotFoundException(Type modelType, int modelId)
+            : base("{0} with ID {1} does not exist", modelType.Name, modelId)
+        {
+
+        }
+    }
+}