doc: check for errors in 'listen' event

In the docs we typically check for errors and surface them. This
is IMO a good idea and good practice. This PR adds a check for
errors in three places in the `net` docs where it was missing.

PR-URL: https://github.com/nodejs/node/pull/4834


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com&gt;>