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: #17709

Two of _http_outgoing.js's errors were migrated to
the internal/errors.js system. This commit
adapts _http_outgoing.js's tests accordingly.

Refs: #17709

_http_outgoing.js's tests perform error-checks. They ensure that
appropriate errors are thrown in the appropriate circumstances.

A few error-checks were bloated. They used unnecessary intermediate
functions. E.g. there were unnecessary combinations of
`common.mustCall()` & `common.expectsError()` and
`common.expectsError()` & `assert.throws()`.

This commit removes those unnecessary intermediate functions.
This simplifies the given error-checks.