code: def _validate_server_url(self):
          
          try:
              request = requests.head(self.server_url)
              if request.status_code >= 400:
                  raise InvenioConnectorServerError(
                      "Unexpected status code '%d' accessing URL: %s"
                      % (request.status_code, self.server_url))
          except (InvalidSchema, MissingSchema) as err:
              raise InvenioConnectorServerError(
                  "Bad schema, expecting http:// or https://:\n %s" % (err,))
          except ConnectionError as err:
              raise InvenioConnectorServerError(
                  "Couldn't establish connection to '%s':\n %s"
                  % (self.server_url, err))
          except InvalidURL as err:
              raise InvenioConnectorServerError(
                  "Invalid URL '%s':\n %s"
                  % (self.server_url, err))
          except RequestException as err:
              raise InvenioConnectorServerError(
                  "Unknown error connecting to '%s':\n %s"
                  % (self.server_url, err))
docs: Validates self.server_url