As your actually giving them a real Senate mission I'm surprised that they don't follow the same system and generate the mission failed message when the ten turns are up.
Have you found where the number of turns is defined for the mission?
Either way, for failure you could try running a console command to test the message, have a look at the list here. The names for the events are in descr_events_images and enums, there's one there called senate_mission_failed. Try that one.
Bookmarks