command: use exit code 2 if arguments are invalid

[davidchambers]

There are now three possible exit codes:

• 2 – invalid arguments
• 1 – test failures
• 0 – success

[davidchambers]

https://github.com/tj/commander.js/releases/tag/v4.0.0:

.exitOverride() allows override of calls to process.exit for additional error handling and to keep program running (tj/commander.js#1040)

[davidchambers]

Aldwin pointed me to https://tldp.org/LDP/abs/html/exitcodes.html which suggests avoiding exit codes such as 1 and 2.

Let's give the matter more thought then update exit codes comprehensively.