We don't want to regenerate a backup if scheduled backups are disabled, so
we now do a check inside of the DeferredTask for the scheduling pref before
attempting a regeneration.
The prior patches in this stack cause us to attempt to delete any known
backups when doing certain clearing operations. If a backup destination
folder was not configured or is pointing at a non-existant directory,
this can cause us to throw. This is non-fatal but annoying.
This patch is some belt-and-suspenders which changes the behaviour so that:
1. Deletion is not attempted if scheduled backups are not enabled.
2. The backup destination pref is checked for non-empty values before
attempting to check its existence.
3. The existence check is wrapped in a try/catch in the event that
the destination pref is set to an invalid path for the OS.
Differential Revision: https://phabricator.services.mozilla.com/D221478
69fe481887