Micro AIs: add check for valid recruiting and error message

This now checks whether the recruiting intended to be done with
ai.recruit is possible and displays an error message if it is not.

data/ai/lua/ai_helper.lua

@@ -173,6 +173,17 @@ function ai_helper.checked_move(ai, unit, x, y)
     ai_helper.checked_move_core(ai, unit, x, y, 'ai.move')
 end
 
+function ai_helper.checked_recruit(ai, unit_type, x, y)
+    local check = ai.check_recruit(unit_type, x, y)
+
+    if (not check.ok) then
+        ai_helper.checked_action_error('ai.recruit', check.status)
+        return
+    end
+
+    ai.recruit(unit_type, x, y)
+end
+
 function ai_helper.checked_stopunit_all(ai, unit)
     local check = ai.check_stopunit(unit)
 

data/ai/micro_ais/cas/ca_recruit_random.lua

@@ -103,7 +103,7 @@ end
 function ca_recruit_random:execution(ai, cfg)
     -- Let this function blacklist itself if the chosen recruit is too expensive
     if wesnoth.unit_types[recruit].cost <= wesnoth.sides[wesnoth.current.side].gold then
-        ai.recruit(recruit)
+        AH.checked_recruit(ai, recruit)
     end
 end