Bug 2032 NBI leaves files orphaned in fsmongo

If any form of error occurs when uploading a package, including uploading
a package that already exists, the NBI will leave the unpacked descriptor's
files in fsmongo.  This change tracks fsmongo changes and rolls them back
on error.

Fixes Bug 20332

Change-Id: I4513747c10e469a01a1cef8b89986eb4b6ad59ae
Signed-off-by: beierlm <mark.beierl@canonical.com>