check-boards: show the correct format in the error message

[jepler]

The message will now say e.g.,

2021-1-21 is an invalid date for example-board. The format must be YYYY-MM-DD, using leading zeros if necessary

[makermelissa]

The leading zeroes are actually optional, so the correct date format is either YYYY-M-D or YYYY-MM-DD.

[jepler]

The yaml timestamp type is documented as requiring a 4-digit year and 2-digit day/month. We previously made changes to ensure that dates are always written as YYYY-MM-DD, because of how jekyll treats front matter.

Having some dates in a different format was one of the problems with correctly ordering the RSS feed. We need to flag this as an error during our CI process, because when the incorrect format loads as a string, the ordering based on the date field is not what is intended.

Detecting the problem but not showing what format was actually required caused trouble for a contributor.

>>> import yaml
>>> yaml.safe_load('[2020-1-1, 2020-01-01]')
['2020-1-1', datetime.date(2020, 1, 1)]

[makermelissa]

Fair enough. Looks like the checks are more strict than the JavaScript.