code: def set_error_page(self, loadbalancer, html):
          
          uri = "/loadbalancers/%s/errorpage" % utils.get_id(loadbalancer)
          req_body = {"errorpage": {"content": html}}
          resp, body = self.api.method_put(uri, body=req_body)
          return body
docs: A single custom error page may be added per account load balancer
          with an HTTP protocol. Page updates will override existing content.
          If a custom error page is deleted, or the load balancer is changed
          to a non-HTTP protocol, the default error page will be restored.