And another Deadwood snapshot today

I got rid of the cryptic-looking error message and now have a fairly clear error message posted when people do something like upstream_servers["example.com"] = "192.168.99.98". I also make it clear in the documentation that the domain name in upstream_servers must end in a dot.

Took me maybe ten minutes, but makes life a lot easier for my users and potential users. Good code is easy to set up and get going. Deadwood suffers here because it uses a CLI instead of a GUI, but at least it's not a case of the documentation being deficient or the error messages being useless.