/**
 * Retrieve value of a field of an email template
 *
 * @param string $templateId Name of an email template
 * @param string $fieldName Name of a field value of which to return
 * @return string
 * @throws UnexpectedValueException
 */
protected function _getInfo($templateId, $fieldName)
{
    $data = $this->_dataStorage->get();
    if (!isset($data[$templateId])) {
        throw new UnexpectedValueException("Email template '{$templateId}' is not defined.");
    }
    if (!isset($data[$templateId][$fieldName])) {
        throw new UnexpectedValueException(
            "Field '{$fieldName}' is not defined for email template '{$templateId}'."
        );
    }
    return $data[$templateId][$fieldName];
}