Display a better error message when config.xml is corrupt

NzbDrone.Core/Configuration/InvalidConfigFileException.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using NzbDrone.Common.Exceptions;
+
+namespace NzbDrone.Core.Configuration
+{
+    public class InvalidConfigFileException : Exception
+    {
+        public InvalidConfigFileException(string message, Exception innerException) : base(message, innerException)
+        {
+        }
+    }
+}