ai_helper checked actions: move error string into call to error()

… rather than making it a separate message.

data/ai/lua/ai_helper.lua

@@ -169,8 +169,7 @@ end
 
 function ai_helper.checked_action_error(action, error_code)
     if wesnoth.game_config.debug then
-        wesnoth.message('Lua AI error', action .. ' could not be executed. Error code: ' .. error_code)
-        error()
+        error(action .. ' could not be executed. Error code: ' .. error_code)
     end
 end