- Use early return
- Add exception_string read-only root

Change-Id: Iba935a4a0308d21ced2693047fdf217b7f9e62f4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3097884Reviewed-by: Patrick Thier <pthier@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76335}