Examine how --exclude handles unresolvable names

[dmiller-nmap]

Initial bug report/enhancement request: http:https://seclists.org/nmap-dev/2013/q4/293

Current behavior is safest: you asked to exclude this name, but Nmap can't tell what address to exclude, so we quit immediately. But perhaps we could be more flexible if the user requests it?

In any case, we should document how Nmap handles unresolvable names and why.