Add reference to the -help command when running without options

[oleg-nenashev]

The tool fails when running without options. It would be better to print a message about the -help command in such case.

onenashev@LAPTOP-25K65LOS:/c/Users/Oleg/Documents/jenkins/demo/demo-jenkins-config-as-code$ java -jar jenkins-plugin-manager-1.0.2.jar
No .txt or .yaml file containing list of plugins to be downloaded entered.
No directory to download plugins entered. Will use default of /usr/share/jenkins/ref/plugins
No CLI option or environment variable set for update center, using default of https://updates.jenkins.io/update-center.actual.json
No CLI option or environment variable set for experimental update center, using default of https://updates.jenkins.io/experimental/update-center.actual.json
No CLI option or environment variable set for incrementals mirror, using default of https://repo.jenkins-ci.org/incrementals
No war entered. Will use default of /usr/share/jenkins/jenkins.war
Unable to create plugin directory

[mikepain]

Hi @oleg-nenashev , so is the requirement that if there is one or more parameter missing, you just want one message displayed that tells the user to look at the help?