Better error reporting when challenge/response fails

Part of requesting a SSL certificate from Let's Encrypt requires the web sites to serve the challenge file. Many of the bugs reported for this site-extension have been that this process fails or is hard to debug. This release tries to solve that by showing an informative exception when this process fails.
The exception looks like this, and provides a link to the URL that should be browsable, and another link to Let's Encrypt with their error log.