Retrieve and validate certificate chain.

        This method validates if the URL is valid and loads and
        validates the certificate chain, before returning it.

        :param cert_url: URL for retrieving certificate chain
        :type cert_url: str
        :return The certificate chain loaded from the URL
        :rtype cryptography.x509.Certificate
        :raises: :py:class:`VerificationException` if the URL is invalid,
            if the loaded certificate chain is invalid

def _retrieve_and_validate_certificate_chain(self, cert_url):
        # type: (str) -> Certificate
        """Retrieve and validate certificate chain.

        This method validates if the URL is valid and loads and
        validates the certificate chain, before returning it.

        :param cert_url: URL for retrieving certificate chain
        :type cert_url: str
        :return The certificate chain loaded from the URL
        :rtype cryptography.x509.Certificate
        :raises: :py:class:`VerificationException` if the URL is invalid,
            if the loaded certificate chain is invalid
        """
        self._validate_certificate_url(cert_url)

        cert_chain = self._load_cert_chain(cert_url)
        self._validate_cert_chain(cert_chain)
        return cert_chain