Function exit_code_for_clap_error 

fn exit_code_for_clap_error(err: &Error) -> ExitCode

Maps a clap error kind to the canonical ExitCode.

Only DisplayHelp and DisplayVersion (explicit --help / --version) are successes; everything else (parse errors, missing subcommand, missing required arg, conflict, DisplayHelpOnMissingArgumentOrSubcommand) is Usage.