Error out if PNG or GRID is requested for a MapConfig with no "mapnik" layers
Error out if PNG or GRID is requested for a MapConfig with no "mapnik" layers

CartoDB/Windshaft,manueltimita/Windshaft,HydroLogic/Windshaft,CartoDB/Windshaft,wsw0108/Windshaft,manueltimita/Windshaft,HydroLogic/Windshaft,wsw0108/Windshaft,HydroLogic/Windshaft,CartoDB/Windshaft,manueltimita/Windshaft,wsw0108/Windshaft