Consider narrower error type between try_new_from_codes and try_from_fields

[sffc]

issue: these new variants are only returned by from-fields, not by constructors on Date (which is what this type is for). please lets not have catch-all errors again

Originally posted by @robertbastian in #6910 (comment)

[sffc]

Also consider making an error module as suggested by @Manishearth in #7012 (comment)