duplicity incorrectly thinks backups are uploaded with --asynchronous-upload and gdocs

[Zhongfu Li]

When duplicity is used to back up to Google Drive with the gdocs:// URL scheme and --asynchronous-upload, duplicity reports that the backups were uploaded correctly even with the internet connection disconnected.

The gdocs backend should instead report back, telling duplicity that the upload has failed and stopping the backup as this can lead to incomplete backup chains when the internet connection is later reconnected.

root@polaris:~# lsb_release -rd
Description: Ubuntu 15.04
Release: 15.04

root@polaris:~# apt-cache policy duplicity
duplicity:
  Installed: 0.7.01-1ubuntu1
  Candidate: 0.7.01-1ubuntu1
  Version table:
 *** 0.7.01-1ubuntu1 0
        500 http://ftp.iinet.net.au/pub/ubuntu/ vivid/main amd64 Packages
        100 /var/lib/dpkg/status

root@polaris:~# duply dir backup
================================================================================
duplicity 0.7.01 (January 11, 2015)
Args: /usr/bin/duplicity --archive-dir /tmp/.duply/cache --name duply_dir --encrypt-key 01234567 --sign-key 01234567 --verbosity 5 --exclude-if-present .duplicity-ignore --full-if-older-than 1M --volsize 100 --archive-dir /tmp/.duply/cache --allow-source-mismatch --asynchronous-upload --force --exclude-globbing-filelist /root/.duply/dir/exclude /dir gdocs://<email address hidden>/backup
Linux polaris 4.0.0-040000-generic #201504121935 SMP Sun Apr 12 23:36:33 UTC 2015 x86_64 x86_64
/usr/bin/python 2.7.9 (default, Apr 2 2015, 15:33:21)
[GCC 4.9.2]
================================================================================
...
A dir/file_1
A dir/file_2
AsyncScheduler: scheduling task for asynchronous execution
Writing duplicity-full.20150525T120004Z.vol1360.difftar.gpg
Attempt 2 failed. gaierror: [Errno -2] Name or service not known
Writing duplicity-full.20150525T120004Z.vol1360.difftar.gpg
Attempt 3 failed. gaierror: [Errno -2] Name or service not known
Writing duplicity-full.20150525T120004Z.vol1360.difftar.gpg
Attempt 4 failed. gaierror: [Errno -2] Name or service not known
Writing duplicity-full.20150525T120004Z.vol1360.difftar.gpg
Processed volume 1361
Writing duplicity-full.20150525T120004Z.vol1361.difftar.gpg
Attempt 1 failed. gaierror: [Errno -2] Name or service not known
A dir/file_3
A dir/file_4
...