OSError if duplicity fails after the last volume but before signatures are transferred

[Liraz]

If you hit CTRL-C after the last volume is created but before the signature file, trying to invoke duplicity again will fail with two
kinds of errors. The first type of error is here:

https://bugs.launchpad.net/duplicity/+bug/556609

The second type of error:

Deleting local
/var/cache/duplicity/6ba373041a8ebe6cdcdbe79e5d16ad38/duplicity-full-signatures.20100729T034328Z.sigtar.gz
(not authoritative at backend).
Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1251, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1244, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1145, in main
    sync_archive()
  File "/usr/bin/duplicity", line 957, in sync_archive
    remove_local(fn)
  File "/usr/bin/duplicity", line 872, in remove_local
    os.unlink(del_name)
OSError: [Errno 2] No such file or directory:
'/var/cache/duplicity/6ba373041a8ebe6cdcdbe79e5d16ad38/duplicity-full-signatures.20100729T034328Z.sigtar.gz'