public function getError()
    {
        if ($this->errorCode !== null || $this->success === false) {
            return [
                'code'    => $this->errorCode === null ? self::UNKNOWN_ERROR_CODE : $this->errorCode,
                'message' => $this->errorMessage === null ? self::UNKNOWN_ERROR_MSG : $this->errorMessage
            ];
        } else {
            return null;
        }
    }