S3: cleanup if key.get_contents_to_filename fails

if key.get_contents_to_filename fails, it leaves an empty file and open file descriptor behind.  Instead, remove the file (since we created it, or at least truncated it to 0 length) and close the file descriptor.
Merge branch 'hashbackup-develop' into develop

* hashbackup-develop:
  S3: cleanup if key.get_contents_to_filename fails