Add a test for missing files.
Make the logs optional.
Add TODO comments.
Log missing files.
Add one more TODO.
Merge pull request #1258 from ericsnowcurrently/backups-missing-logs

Make log files optional in a backup archive.

https://bugs.launchpad.net/juju-core/+bug/1398448

(Review request: http://reviews.vapour.ws/r/572/)