Clean up flight plan code.

Split the oversized file into one per plan type. This also moves the
layout responsibility out of the oversized FlightPlanBuilder and into
each flight plan type file.

game/ato/flightplans/invalidobjectivelocation.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+
+from game.ato import FlightType
+from game.ato.flightplans.planningerror import PlanningError
+from game.theater import MissionTarget
+
+
+class InvalidObjectiveLocation(PlanningError):
+    """Raised when the objective location is invalid for the mission type."""
+
+    def __init__(self, task: FlightType, location: MissionTarget) -> None:
+        super().__init__(f"{location.name} is not valid for {task} missions.")