Watcher: croneval tool prints error on valid one time expression

[spinscale]

Using the croneval CLI tool, this spits an error to a perfectly valid one-time cron expression (tested on 6.3.2)

./bin/elasticsearch-croneval "0 3 23 8 9 ? 2019"
Valid!
Now is [Thu, 9 Aug 2018 07:54:34]
Here are the next 10 times this cron expression will trigger:
1.	Mon, 9 Sep 2019 01:03:00
ERROR: 2.	 Could not compute future times since [Mon, 9 Sep 2019 01:03:00] (perhaps the cron expression only points to times in the past?)

Instead of printing an error, we should just print the first line and exit and be good, as a one time cron expression is perfectly fine.

[elasticmachine]

Pinging @elastic/es-core-infra