Bug#618920: debootstrap: needs more robust download error handling

[Michael Gilbert <michael.s.gilbert@gmail.com>]

I probably should have mentioned that these errors are much more
likely within d-i for some reason.  The best way to reproduce this is
to do an installation up through partitioning, then do:

$ debootstrap wheezy /target/wheezy-chroot
http://snapshot.debian.org/archive/debian/20110318T093210Z

At least a few package downloads should fail from that mirror, and
they'll be different every time.

Executing the same command in a full system seems to work fine for
that exact same mirror, so maybe its some other variability within d-i
that is the cause of this issue.

Best wishes,
Mike