Exit code control from built-in handlers

[remkop]

Commonly, successful execution of a command should result in a zero exit code and unsuccessful invocation should result in a non-zero exit code.

The java executable does not return a non-zero exit code if the application exits because of an uncaught throwable. It’s up to the application to call System.exit(int) if control over the exit code is desired.

Picocli has a “success” path and three separate “failure” paths:

• InitializationException if misconfigured
• ParameterException if user input is invalid
• ExecutionException if user input was valid but the IParseResultHandler threw an exception
• No errors

Consider providing API to let applications control the exit code for all of the above.

See also discussion under #218 for API ideas on how to accomplish this.

[remkop]

Note to self: look at http://stefanbirkner.github.io/system-rules/#ExpectedSystemExit to help with testing

[remkop]

Text for release notes:

• [251] New feature: exit code support. The built-in parse result handlers (RunFirst, RunLast and RunAll) and exception handler (DefaultExceptionHandler) can now specify an exit code. If specified, the handler terminates the JVM with the specified status code when finished. Custom handlers can extend AbstractHandler to inherit this behaviour.