errors: minor (SystemError) refactoring

This removes the former default values and the spread arguments
usage. That was unnecessary and now it does only what is necessary.
The `message` function got renamed to `getMessage` to outline that
it is actually a function and a helper function was inlined into
the SystemError constructor as it was only used there.

PR-URL: https://github.com/nodejs/node/pull/20337


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>