errors: make properties mutable

Userland code can break if it depends on a mutable `code` property for
errors. Allow users to change the `code` property but do not propagate
changes to the error `name`.

Additionally, make `message` and `name` consistent with `Error` object
(non-enumerable). Test that `console.log()` and `.toString()` calls on
internal `Error` objects with mutated properties have analogous results
with the standard ECMAScript `Error` objects.

PR-URL: https://github.com/nodejs/node/pull/15694
Fixes: https://github.com/nodejs/node/issues/15658


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>