improve pngload error messages too
improve pngload error messages too

see https://github.com/libvips/libvips/issues/1578

jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips