code: function getErrorStatusCode (err) {
    // check err.status
    if (typeof err.status === 'number' && err.status >= 400 && err.status < 600) {
      return err.status
    }
  
    // check err.statusCode
    if (typeof err.statusCode === 'number' && err.statusCode >= 400 && err.statusCode < 600) {
      return err.statusCode
    }
  
    return undefined
  }
docs: Get status code from Error object.
  
  @param {Error} err
  @return {number}
  @private