Improve the "selfVerify failed" error message

[Ayesh]

Hi Niklas,
I use acme-client extensively, in many of my servers, and couldn't be happier with it. Thanks for the awesome work you have put in!

I noticed some of the recent Github issues are about the users getting selfVerify validation errors. I was wondering if it would be better if we expose more information to help them debug why it failed.

Most Apache configurations block dot-directories with a 403 error, and if we show the response status code returned from Artax (404, 403, 500, etc), it gives a hint if the web server is blocking the request. 404 requests could indicate a wrong doc root used in the issue command, likewise.

Thanks.

[kelunik]

@Ayesh Definitely a good idea. I haven't really worked on the client for a while now, it should really get an update there.