Handle runway damage in the debrief.

Apparently we were already getting this info because it's a unit like
any other according to the event system, so if runways were actually
sufficiently damaged we'd emit a ton of exceptions.

This doesn't do anything yet, but tracks the damage state. Will add the
ability to repair them next, and then finally make the damage render the
runway inoperable.

game/event/event.py

@@ -117,6 +117,9 @@ class Event:
 
         logging.info("Commiting mission results")
 
+        for damaged_runway in debriefing.damaged_runways:
+            damaged_runway.damaged = True
+
         # ------------------------------
         # Destroyed aircrafts
         for loss in debriefing.air_losses.losses: