_checkForMissingMandatoryOptions() {
    // Walk up hierarchy so can call in subcommand after checking for displaying help.
    this._getCommandAndAncestors().forEach((cmd) => {
      cmd.options.forEach((anOption) => {
        if (
          anOption.mandatory &&
          cmd.getOptionValue(anOption.attributeName()) === undefined
        ) {
          cmd.missingMandatoryOptionValue(anOption);
        }
      });
    });
  }
Display an error message if a mandatory option does not have a value.
Called after checking for help flags in leaf subcommand.

@private