src: fix -Wunused-result warning

Fix a compiler warning that was introduced in commit c3dc0e0d
("src: add CollectExceptionInfo & errors.SystemError") by adding
a `.FromJust()` call.

PR-URL: https://github.com/nodejs/node/pull/16726


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>