Previously, a failed backup would always overwrite ${db}.sql.gz, because the bash `>` redirect truncates the file; even if the backup was going to fail. On the next run, the ${db}.prev.sql.gz backup would be overwritten by the bad ${db}.sql.gz. Now, if the backup fails, the ${db}.in-progress.sql.gz is in an unknown state, but ${db}.sql.gz will not be written. On the next run, ${db}.prev.sql.gz (our only good backup) will not be overwritten because ${db}.sql.gz does not exist.