Fail check on non success status code

[clample]

Currently, external links resulting in a 404 error or other non success status codes will still pass the check.

With these changes, the user is now alerted about non success status codes from external links.

[clample]

I'm not sure if an error makes sense for all non 2** response codes. It seems like we might get non 2** response codes for pages that are still accessible from the browser. For example, https://medium.freecodecamp.com/yes-im-56-and-learning-to-code-f33abea6fd4c#.zhqs1isvb gave a 409 error when running the check tool but I was able to access this from the browser.

On the other hand, it might be better to over-report issues when looking for broken links than to let possible issues go unnoticed.

[jaspervdj]

This looks good but I think that 3xx status codes should also be valid, external URLs should be free to be moved around.