nsqlookupd: better API when used as a library

[stephens2424]

for package importers to get error info, and to allow importers avoid a call to os.Exit.

fixes #1003

[mreiferson]

I'm fine with cleaning this up, and I don't particularly care about backwards compatibility (which we haven't traditionally guaranteed when using NSQ as a library).

Instead I would suggest making Main() actually behave appropriately by returning an error and promote the logging and exiting to the package main code paths.

[ploxiln]

looks like this was updated (it helps to comment after updating the PR branch, because sometimes github doesn't notify for that)

[mreiferson]

yea, this LGTM

[mreiferson]

ok, fixed up this commit msg