operator.backup: update wal_prefetch() so that failed downloads are deleted

Resolves the issue in v0.8c1 detailed at:

    https://groups.google.com/forum/#!topic/wal-e/8wwiDkrNLXQ

wherein (standby_mode = off) PostgreSQL recoveries were failing when
--prefetch > 0. Specifically, in cases where do_lzop_get() failed to
fetch an archive file (as would be the case when trying to fetch the
non-existent WAL file following the latest one in the archive) prefetch
was creating an empty archive file and then allowing AtomicDownload to
link that file into place. This causes PostgreSQL's recovery to fail.
PostgreSQL then shuts down and can't be started up until you remove
recovery.conf.

Sorry to not include a test for this; I've had difficulties getting tox up and
running in my environment. My best guess would be to add a test to
test_blackbox.py along the lines of test_wal_fetch_non_existent(). I'm
not certain, however.