Raise NoParkingSlotError instead of RuntimeError when running out of ground spawns.

game/missiongenerator/aircraft/flightgroupspawner.py

@@ -436,7 +436,9 @@ class FlightGroupSpawner:
                 )
                 group.units[1 + i].heading = ground_spawn[0].units[0].heading
             except IndexError as ex:
-                raise RuntimeError(f"Not enough STOL slots available at {cp}") from ex
+                raise NoParkingSlotError(
+                    f"Not enough STOL slots available at {cp}"
+                ) from ex
         return group
 
     def dcs_start_type(self) -> DcsStartType: