src/Main.hs: fix handling of "domain not found" errors.

src/Main.hs: fix handling of "domain not found" errors.

The "not delegated" example in the man page is now failing, because
we're getting back a NameError (failure result) instead of an empty
success result. I'm not sure when this changed -- and my fix is
probably backwards-incompatible -- but I've updated haeredes to look
for the NameError rather than an empty result. The documented example
works again, but I've only tested with v3.0.2 of the "dns" library.