when --delete is used we should leave an empty database in place. If
fail2ban_digest is used via cron the missing DB will cause an error to
be triggered and the admin will be mailed because one cron job failed,
which is not the case actually
Using the dbm module with autodetection is quite broken. Opening a dbm
file with 'r' flag will raise a useless dbm.error, not inherinting
OSError. The reason for the error is lost. When forcing GNU dbm module a
proper exception is raised.
