How to use the drachtio-srf.SipError function in drachtio-srf

const parseUri = require('drachtio-srf').parseUri ;
const SipError = require('drachtio-srf').SipError;
const config = require('config');
const debug = require('debug')('drachtio-rtpengine-webrtcproxy') ;

class CallProcessor {

  /**
   * creates an instance of the call processor.  this is intended to be a singleton instance.
   * you call the 'start' method to kick it off
   */
  constructor(logger) {
    this._logger = logger;

    // we need to track these only so we can fixup REFER messages for attended transfer
    this.calls = new Map() ;
  }

const parseUri = require('drachtio-srf').parseUri ;
const SipError = require('drachtio-srf').SipError;

class Subscriber {

  constructor(logger) {
    this._logger = logger;
  }

  get logger() {
    return this._logger;
  }

  start(srf, registrar) {
    srf.subscribe((req, res) => {

      this.logger.info(`UAC subscribing: ${req.protocol}/${req.source_address}:${req.source_port}`) ;