This checks if the next middleware to be called is nil, and if so returns
ServerFailure and an error. This makes the next calling more robust and
saves some lines of code.
Also prefix the error with the name of the middleware to aid in
debugging.
* middleware/httproxy: implement debug queries
Not too useful at the moment, but o-o.debug queries are supported
and return the Comment from dns.google.com.
Note that this is not always set.
* improve documentation
* Testing cleanups