Traditionally we have tried to download all and any packages we can
find.  If we have any packages we just assume that what we got is a
consistent set and use it.  This leads to incomplete sets being
committed on network failure.

Firstly detect and differentiate transport errors and valid missing
packages.  Secondly switch to analysing known good package set
combinations; this relies on the presumption that the publisher
only publishes all or none of a binary package set.  This lets us
throw errors when we are unable to find an internally consistent
set of packages.

BugLink: http://bugs.launchpad.net/bugs/1806380Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>