lib: migrate _http_outgoing.js's remaining errors

A couple of lib/_http_outgoing.js's errors were still in the
"old style": `throw new Error(<some message here>)`.

This commit migrates those 2 old style errors to the "new style":
internal/errors.js's error-system.

In the future, changes to these errors' messages won't break
semver-major status. With the old style, changes to these errors'
messages broke semver-major status. It was inconvenient.

Refs: https://github.com/nodejs/node/issues/17709
PR-URL: https://github.com/nodejs/node/pull/17837


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