Reject intervals that are out of order

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

cronexpr_parse.go

@@ -438,6 +438,9 @@ func genericFieldParse(s string, desc fieldDescriptor) ([]*cronDirective, error)
 			directive.first = desc.atoi(snormal[pairs[2]:pairs[3]])
 			directive.last = desc.atoi(snormal[pairs[4]:pairs[5]])
 			directive.step = 1
+			if directive.last < directive.first {
+				return nil, fmt.Errorf("invalid interval %s (normalized to %d-%d)", snormal, directive.first, directive.last)
+			}
 			directives = append(directives, &directive)
 			continue
 		}