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
the ca9eb71 behavior.

Fixes #7731.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Signed-off-by: Fedor Indutny <fedor@indutny.com>