Comment 1 for bug 495179

I like this solution more than my proposal in my comment on the other bugreport. But I still think it is not optimal because there is no reason to distinguish between different reason of failure. The client will always get the same result back no matter if this event was blocked by an extension, blocked because the event already had an id, blocked because the client tried to add an event without a subject, and so on.
Is it possible to send negative integers over dbus, so we can define
  v >= 0 -> id of an event
  v < 0 -> error

We also predefine some error values as constants.