If an interval if out of order, it ends up emitting 0000-00-00 00:00:00
as the "next" time, which is undesirable. This patch updates Parse to
simply reject such intervals.
Since the rejection reason might not be super obvious (the range 6-7 is
actually translated to 6-0, which makes it invalid), let's also print
the original vs. normalized form for clarity.
See: https://github.com/aptible/supercronic/issues/63
Thomas Orozco