Guard against invalid schedule loading

[taf2]

If a bad crontab is published to a schedule this will blow up and cause the process to become corrupted. This causes lots of schedules to be missed.

[meatballhat]

@taf2 Hello! Can you rebase against master, and please edit your commit message to remove the profanity? Thank you! 😸

[iloveitaly]

@taf2 friendly reminder on this PR!