This is an important consideration: if BGSAVE fails and poisons the
database, snapshotting can and should too. But this is debatable in some
parts. For example, users may configure snapshots to be on a network
file system (symlinked maybe) and this can fail.
Now in some cases, this failure 'may be acceptable'. This commit adds a
way to customize this behavior through the `failsafe` key in the
snapshots section of the cfg file and through the --stop-write-on-fail
option passed to `skyd` on startup. However, BGSAVE remains unchanged:
it will always poison the database if it fails. If the user doesn't want
this, they can simply disable BGSAVE.