_exit(exitCode, code, message) {
    if (this._exitCallback) {
      this._exitCallback(new CommanderError(exitCode, code, message));
      // Expecting this line is not reached.
    }
    process.exit(exitCode);
  }
Call process.exit, and _exitCallback if defined.

@param {number} exitCode exit code for using with process.exit
@param {string} code an id string representing the error
@param {string} message human-readable description of the error
@return never
@private