fail if both level and associated image are specified
fail if both level and associated image are specified

rather than silently ignoring the level.

jcupitt/libvips,wishabi/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,mcanthony/libvips,wishabi/libvips,leiyangyou/libvips,jcupitt/libvips,leiyangyou/libvips,jcupitt/libvips,mcanthony/libvips,leiyangyou/libvips,leiyangyou/libvips,lovell/libvips,wishabi/libvips,wishabi/libvips,lovell/libvips,mcanthony/libvips,mcanthony/libvips