A worker might be terminated while creating a new Realm. While this was
handled mostly correctly already, a DCHECK was places slightly too
early, which is fixed by this CL.
Also, we avoid printing an error message if we fail to install an
extension due to isolate termination. As this is externally triggered,
it's not really an error condition.

R=jkummerow@chromium.org

Bug: chromium:1313475
Change-Id: I67b7fd27002d9b9a33439378d8336fefb2a2371a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3571811Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79825}