Exit if mysqldump is not found

[fauust]

Hi!
I am moving local storage to s3 (https://garagehq.deuxfleurs.fr) using your tool.

So far so good but I would suggest that the script exit if the DB backup did not worked as expected (or verify in the script that requirements are available):

set  maintenance:mode --on:
Maintenance mode enabled

database backup...sh: mysqldump: not found

(to restore: mysql -u nextcloud -p nextcloud < backup.sql)

backup config.php...
connect to S3...

I am not good at PHP so I cannot propose a PR.