doc: improvements to errors.markdown copy

General improvements to errors.markdown including
improved/revised examples

PR-URL: https://github.com/nodejs/node/pull/4454


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>