src: add .FromJust(), fix -Wunused-result warnings

Missed while reviewing 1fde98bb ("v8: expose new V8 serialization API.")

PR-URL: https://github.com/nodejs/node/pull/12118
Refs: https://github.com/nodejs/node/pull/11048


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>