Introduction Protocol does not abort once in FINISHED state
While writing introduction protocol attacks for #627 (closed) I noticed that introducees do not handle ABORT messages once they are in the FINISHED state.
This can lead to a situation where the protocol is aborted by Bob due to an invalid MAC, but Alice has not only added Bob already, but also notified the UI of the successful addition of Bob. Do we also forcefully remove a contact that has already been added and activated? If so, do we need to introduce a new event to inform the user about that.