TBTR: Refactor template replacement code

Adjust conditions for re-using existing engines in depot
Reduce cost of searching for existing engines in depot
Reduce cost of vehicle chain membership tests
Improve replacement command error handling
Tidy up code in general

src/saveload/tbtr_template_veh_sl.cpp

@@ -68,7 +68,7 @@ static void Load_TMPLS()
 	int index;
 
 	while ((index = SlIterateArray()) != -1) {
-		TemplateVehicle *tv = new (index) TemplateVehicle(); //TODO:check with veh sl code
+		TemplateVehicle *tv = new (index) TemplateVehicle();
 		SlObject(tv, GTD());
 	}
 }