I'm pretty sure the "Repair stopped" message comes from movement of either the spider repairing or vehicle being repaired. I think you get the same message if a vehicle passing by forces a spider repairing a structure to move as well.
As mentioned, repair vehicles and spiders can only repair a vehicle up to 50%. Once the vehicle is repaired up to that limit, you will get the message "Vehicle Reparied" if I'm not mistaken
Edit: You will also get an error message - probably the "Repair stopped", if you run out of ore while repairing something.