added JsonErrorFilter to automatically handle failing ajax calls.

NzbDrone.Web/Filters/JsonErrorFilter.cs

@@ -0,0 +1,24 @@
+using System.Linq;
+using System.Web.Mvc;
+using NzbDrone.Web.Models;
+
+namespace NzbDrone.Web.Filters
+{
+    public class JsonErrorFilter : FilterAttribute, IExceptionFilter
+    {
+        private readonly string _errorTitle;
+
+        public JsonErrorFilter(string errorTitle)
+        {
+            _errorTitle = errorTitle;
+        }
+
+        public void OnException(ExceptionContext filterContext)
+        {
+            filterContext.Result = NotificationResult.Error(_errorTitle, filterContext.Exception.Message);
+            filterContext.ExceptionHandled = true;
+        }
+
+
+    }
+}