Fixed a bug in Intervention

https://github.com/OpenRA/OpenRA/pull/5483#issuecomment-44781904
broke the game.

mods/ra/maps/intervention/intervention.lua

@@ -234,7 +234,7 @@ WorldLoaded = function()
 
 	sovietObjective = soviets.AddPrimaryObjective("Destroy the village.")
 	villageObjective = player.AddPrimaryObjective("Save the village.")
-	beachheadObjective = player.AddSecondaryObjective("Get your MCV to the main island.")
+	beachheadObjective = player.AddPrimaryObjective("Get your MCV to the main island.")
 
 	beachheadTrigger = false
 	Trigger.OnExitedFootprint(BeachheadTrigger, function(a, id)
@@ -244,6 +244,17 @@ WorldLoaded = function()
 			player.MarkCompletedObjective(beachheadObjective)
 
 			captureObjective = player.AddPrimaryObjective("Locate and capture the enemy's Air Force HQ.")
+
+			if AirForceHQ.IsDead then
+				player.MarkFailedObjective(captureObjective)
+				return
+			end
+			if AirForceHQ.Owner == player then
+				player.MarkCompletedObjective(captureObjective)
+				player.MarkCompletedObjective(villageObjective)
+				return
+			end
+
 			Trigger.OnCapture(AirForceHQ, function()
 				Trigger.AfterDelay(DateTime.Seconds(3), function()
 					player.MarkCompletedObjective(captureObjective)
@@ -252,6 +263,8 @@ WorldLoaded = function()
 			end)
 			Trigger.OnKilled(AirForceHQ, function() player.MarkFailedObjective(captureObjective) end)
 
+			Actor.Create("mainland", true, { Owner = player })
+
 			Trigger.AfterDelay(BaseFrontAttackInterval, function()
 				Build(BaseFrontAttackUnits, BaseFrontAttack)
 				ParadropSovietUnits()

mods/ra/maps/intervention/map.yaml

@@ -2248,10 +2248,23 @@ Rules:
 	E6:
 		Buildable:
 			Prerequisites: ~disabled
-	HPAD:
+	TENT:
+		Buildable:
+			Prerequisites: anypower, ~structures.allies, ~techlevel.infonly, mainland
+	DOME:
+		Buildable:
+			Prerequisites: proc, ~techlevel.medium, mainland
+	WEAP:
+		Buildable:
+			Prerequisites: proc, ~techlevel.low, mainland
 		ProvidesCustomPrerequisite:
 			Prerequisite: givefix
-	WEAP:
+	MAINLAND:
+		Tooltip:
+			Name: Reach the mainland
+		ProvidesCustomPrerequisite:
+			Prerequisite: mainland
+	HPAD:
 		ProvidesCustomPrerequisite:
 			Prerequisite: givefix
 	FIX: