export: add meaningful exit code

Currently ‘gnt-backup export’ always returns exit code zero, even in the
face of complete failure during backup (only failure to stop/start the
instance will cause job failure and thus non-zero exit code). This is
bad, since one cannot script the backup.

This patch adds some simple results from the LU so that the command line
script can return good exit code. It will:
  - return zero for full success (snapshot removal errors are ignored
    though)
  - return one for full failure (finalize export failure or all disks
    failure)
  - return two for partial failure (some disks backed up, some not)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>