code: protected int handleError(Exception exception, CliParser parser) {
  
      int exitCode;
      AppendableWriter writer = new AppendableWriter(this.standardError);
      PrintWriter printStream = new PrintWriter(writer);
      if (exception instanceof CliException) {
        // TODO: NLS
        printStream.println("Error: " + exception.getMessage());
        printStream.println();
        printStream.flush();
        printHelp(parser);
        exitCode = 1;
      } else {
        printStream.println("An unexpected error has occurred:");
        exception.printStackTrace(printStream);
        exitCode = -1;
      }
      printStream.flush();
      return exitCode;
    }
docs: This method is invoked if an {@link Exception} occurred. It implements how to handle the error. You may override to
  add a custom handling.
  
  @param exception is the actual error that occurred.
  @param parser is the {@link CliParser}.
  @return the error-code (NOT {@code 0}).