exitOverride(fn) {
    if (fn) {
      this._exitCallback = fn;
    } else {
      this._exitCallback = (err) => {
        if (err.code !== 'commander.executeSubCommandAsync') {
          throw err;
        } else {
          // Async callback from spawn events, not useful to throw.
        }
      };
    }
    return this;
  }
Register callback to use as replacement for calling process.exit.

@param {Function} [fn] optional callback which will be passed a CommanderError, defaults to throwing
@return {Command} `this` command for chaining