dns: send `lookup` c-ares errors to callback

Calling `dns.lookup` with arguments that generate an error from c-ares
previously sent those errors back to the callback. This commit restores
that behavior.

Fixes joyent/node#7731.
LGTM if tests pass.  Bonus points if you check that error.code is an expected value.  You can return e.g. process.binding('uv').ENOENT from the getaddrinfo stub, then check here that error.code === 'ENOENT'.