No error handler if the request fails

[bfintal]

If you have an error in your request (like you are getting a user using an email address that doesn't exist), then the responseCallback will never be called.

In https://github.com/Freemius/freemius-node-sdk/blob/master/Freemius.js#L414, you can see when there's an error, only a console.log is called.