docs: def validate_href(image_href):
          """Validate HTTP image reference.
  
          :param image_href: Image reference.
          :raises: exception.ImageRefValidationFailed if HEAD request failed or
              returned response code not equal to 200.
          :returns: Response to HEAD request.
          """
          try:
              response = requests.head(image_href)
              if response.status_code != http_client.OK:
                  raise exception.ImageRefValidationFailed(
                      image_href=image_href,
                      reason=("Got HTTP code %s instead of 200 in response to "
                              "HEAD request." % response.status_code))
          except requests.RequestException as e:
              raise exception.ImageRefValidationFailed(image_href=image_href,
                                                       reason=e)
          return response