Support ? in cron expressions

[violuke]

Cron format of 0 0 0 ? * SUN * breaks the runner.

We should log an issue, but not break everything if a cron format provided is invalid.

Additionally, can we support a ? to be the same as a *?

Explanation from SO

*("all values") - used to select all values within a field. For example, "" in the minute field means *"every minute".

? ("no specific value") - useful when you need to specify something in one of the two fields in which the character is allowed, but not the other. For example, if I want my trigger to fire on a particular day of the month (say, the 10th), but don't care what day of the week that happens to be, I would put "10" in the day-of-month field, and "?" in the day-of-week field. See the examples below for clarification.

[violuke]

Done in e79fba0