If apt-mirror is unable to download deb files (eg, source mirror is
temporarily missing files, maybe it has bad update logic), apt-mirror
still goes ahead an moves the source mirrors listing files from "skeleton"
to "mirror". This creates an inconsistent local mirror.

apt-mirror should check for errors after downloading the .deb files
(perhaps by re-running the "Proceed indexes" logic).

If this check fails then apt-mirror should fail with a warning, telling you
to retry with one of the following (new) options: