The web frontend for mailman3 is built on django. Django has a fairly
robust caching system to speed up page requests. Mailman3 has configured
Django to use the DiskCache caching system which caches on disk under
/var/lib/mailman/web-data/diskcache. This cache uses sqlite databases
under the hood and unfortunately the files come and go and race our
backup system. When the file disappears under the backup system during
backups we record an error.
Since these are caches that will be rebuilt through normal usage we
don't actually need to back them up. Exclude this entire path from
backups to avoid unwanted errors since these backups are simply not
necessary.
Change-Id: I9a5410fd9f952511c52268f06629e9bbaa20d947
1b7ebd9c55