No parsing error when passing an invalid cron expression to `cron` function

[naz]

refs https://github.com/bunkat/later/issues/174

Problem

As pointed out in referenced issue there is no error detection logic in later.parse.cron function. This introduced an inconsistency with how later.parse.text behave when invalid values are passed in. In case of cron method there is no indication of any type of error if completely invalid string is passed in. Contrary for text method there's error property assigned to returned object when the expression is not parsed.

Possible solution

There are multiple ways I can see this issue can be approached:

1. Leave it as is, but be very specific that making sure the cron expression is valid is on the client
2. Validate cron expression internally and return consistent error just like in case of parsing expression through text function

Personally I'm in favor of second approach as that would allow to unify error handling.

[naz]

To move on with the implementation I've ended up using cron-parser to validate cron expressions before calling later.parse.cron. Might serve as a good reference for parsing logic

[naz]

Had a look into what bree does with this exact problem and saw that it's running cron expression validation against cron-validate.

If the validation for cron expressions is moved into later's layer, I think it will make the logic for both cron and text expressions-to-schedule transformations more consistent.

[hovancik]

Just noticed the same issue. Would make sense to use same as for text parser as right now cron gives me 0 for error all the time and that complicates everything.

[arossert]

I also noticed now that later is not validating cron strings, is there a plan to add this or do I need to validate it manually?