try {
    var json = JSON.parse(yield interwebs.getJSON());
    response.send(json.success);
}
catch (e) {
    if (e instanceof SyntaxError) {
        response.send("Invalid JSON!");
    }
    else {
        throw e;
    }
}


interwebs.getJSON()
    .then(JSON.parse)
    .then(json => response.send(json.success)
    .catch(SyntaxError, e => response.send("Invalid JSON!"));