Some invalid cron expressions show unhelpful error message

[joe4dev]

Entering some invalid cron expressions leads to unhelp error messages.
Note: the feature works when entering valid or obviously incorrect cron expressions, so this is a minor issue.

It happens with 0 24 * * * (hours out of bounds) but not with 0 23 * * * (which is valid)

When trying to create an active schedule:

Failed to update system crontab.

When trying to create an inactive schedule:

`undefined method `+' for nil:NilClass`

This could potentially break the benchmark definitions view if "corrupted" schedules somehow get through validation because Cron2English then fails when rendering the view.

Ideas:

• Refine static regex validator
• Try dynamic validation via some parser (existing Cron2English or https://github.com/alpinweis/cronex)