Added single instance policy

src/NzbDrone.Host/TerminateApplicationException.cs

@@ -4,5 +4,11 @@ namespace NzbDrone.Host
 {
     public class TerminateApplicationException : ApplicationException
     {
+        public TerminateApplicationException(string reason)
+        {
+            Reason = reason;
+        }
+
+        public string Reason { get; private set; }
     }
 }