doc: fixup errors.md

* add semicolons in examples
* fix indentation in code example
* add spaces in code examples
* console.log() -> console.error()
* fix level of headings
* update comment code example
* delete obsolete info and example

Fixes: https://github.com/nodejs/node/issues/11558
PR-URL: https://github.com/nodejs/node/pull/11566


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>