fix openslide_open error reporting for OpenSlide 3.3.0
fix openslide_open error reporting for OpenSlide 3.3.0

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