Elasticsearch version

7.4.2, master

Description of the problem including expected versus actual behavior:

When elasticsearch-croneval is run with a malformed cron pattern, it dumps a stacktrace to the terminal.

The tool ought to simply report that the pattern is malformed and either direct the user to the documentation or print a brief summary of the expected input (I'd vote for the latter).

Steps to reproduce:

1. `bin/elasticsearch-croneval "*"