code: func (r *cmdRun) argErr(shortDesc, usageLine, format string, a ...interface{}) int {
  	if format != "" {
  		fmt.Fprintf(os.Stderr, format+"\n", a...)
  	}
  	fmt.Fprintln(os.Stderr, shortDesc)
  	fmt.Fprintln(os.Stderr, usageLine)
  	fmt.Fprintln(os.Stderr, "\nFlags:")
  	r.Flags.PrintDefaults()
  	return ecInvalidCommandLine
  }
docs: // argErr prints an err and usage to stderr and returns an exit code.