Comment 3 for bug 1507649

> There's no reliable way to recover from a partial install. If something has failed it's safer (and in fact faster) to rebuild the image.

both debootstrap and apt-get are downloading packages at first to process/install them later.

both of them store downloaded files in cache directory. So, it's quite easy and convenient to perform retry.

if we want to retry only on downloading, then both of them are capable of performing downloading only via `--download-only` option.