Fail on non successful network requests

[tomek-ac]

With :network strategy the gem doesn't fail when the stylesheet doesn't exist at given url (404).
This results in emails being sent without styling.

We would like the gem to raise an exception if for whatever reason our CDN styles are not accessible.

In the proposed PR the loader will return a response only when the response was successful (2xx). Otherwise it will return nil which will result in Premailer::Rails::CSSHelper::FileNotFound being raised.