code: func (c *Subcommand) printError(err error) {
  	if _, ok := err.(CommandLineError); ok {
  		fmt.Fprintf(os.Stderr, "Bad command line: %s.\n\n", err)
  		c.Flags.Usage()
  	} else {
  		os.Stderr.WriteString(strings.Join(CollectErrorMessages(err, nil), "\n"))
  		os.Stderr.WriteString("\n")
  	}
  }
docs: // printError prints an error to stderr.
  //
  // Recognizes various sorts of known errors and reports the appropriately.