Added exit on bad crontab

[Dashboy1998]

Context

• People have no way of knowing if their cron expressions are bad.

Choices

• Checking after each addition to the crontab since the test breaks on the first invalid expression
• Also makes it easier to understand which expression broke as I added

Test instructions

1. Start the container and should see new section in output

   palworld-server-test  | *****GENERATING CRONTAB*****
   palworld-server-test  | BACKUP_ENABLED=true
   palworld-server-test  | Adding cronjob for auto backups
   palworld-server-test  | AUTO_UPDATE_ENABLED=true
   palworld-server-test  | Adding cronjob for auto updating
   palworld-server-test  | AUTO_REBOOT_ENABLED=true
   palworld-server-test  | Adding cronjob for auto rebooting
   palworld-server-test  | Cronjobs started
   

2. Start with all cron jobs disabled

   palworld-server-test  | *****GENERATING CRONTAB*****
   palworld-server-test  | No Cronjobs found
   

3. Try with an invalid cron expression

   palworld-server-test  | *****GENERATING CRONTAB*****
   palworld-server-test  | BACKUP_ENABLED=true
   palworld-server-test  | Adding cronjob for auto backups
   palworld-server-test  | AUTO_UPDATE_ENABLED=true
   palworld-server-test  | Adding cronjob for auto updating
   palworld-server-test  | time="2024-02-13T18:01:54Z" level=fatal msg="bad crontab line: 'FOOBAR bash 
   /usr/local/bin/update' (use -debug for details)"
   palworld-server-test exited with code 0
   

Checklist before requesting a review

• I have performed a self-review of my code
• I've added documentation about this change to the README.
• I've not introduced breaking changes.