Failure to sync an image stops g-s-s

When an image download fails, g-s-s comes to a halt. G-s-s should be more robust in handling that ie. it should continue to download other images.

For example, during initial sync the g-s-s log had this error:

DEBUG * 09-07 14:01:13 [PID:10150] * urllib3.connectionpool * "GET /releases/server/releases/raring/release-20140111/ubuntu-13.04-server-cloudimg-amd64-disk
1.img HTTP/1.1" 404 286
ERROR * 09-07 14:01:13 [PID:10150] * root * Exception during do_sync
Traceback (most recent call last):
  File "/usr/share/glance-simplestreams-sync/glance-simplestreams-sync.py", line 286, in <module>
    do_sync(charm_conf)
  File "/usr/share/glance-simplestreams-sync/glance-simplestreams-sync.py", line 152, in do_sync
    tmirror.sync(smirror, path=initial_path)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 88, in sync
    return self.sync_index(reader, path, data, content)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 240, in sync_index
    self.sync(reader, path=epath)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 86, in sync
    return self.sync_products(reader, path, data, content)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 318, in sync_products
    self.insert_item(item, src, target, pgree, ipath_cs)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/glance.py", line 195, in insert_item
    (tmp_path, tmp_del) = util.get_local_copy(contentsource)
  File "/usr/lib/python2.7/dist-packages/simplestreams/util.py", line 445, in get_local_copy
    raise e
IOError: Unable to open http://cloud-images.ubuntu.com/releases/server/releases/raring/release-20140111/ubuntu-13.04-server-cloudimg-amd64-disk1.img. mirrors=[]
INFO * 09-07 14:01:13 [PID:10150] * root * Initial sync attempt done. every-minute cronjob removed.
INFO * 09-07 14:01:13 [PID:10150] * root * sync done.

Even though the raring image was indeed missing, trusty and other images were available. G-s-s should have tried to download those.

PS: g-s-s revno=52

Related bugs:
 * bug 1493395: https://bugs.launchpad.net/simplestreams/+bug/1493395