function sasResponseReceived(id, xhr, isError) {
        var promise = requestPromises[id];

        if (isError) {
            promise.failure("Received response code " + xhr.status, xhr);
        }
        else {
            if (xhr.responseText.length) {
                promise.success(xhr.responseText);
            }
            else {
                promise.failure("Empty response.", xhr);
            }
        }

        delete requestPromises[id];
    }
Sends a GET request to the integrator's server, which should return a Shared Access Signature URI used to
make a specific request on a Blob via the Azure REST API.