Implicitly rethrow the exception when we reach the end of a
try..unwind..end. Also make it a validation error to rethrow
an exception caught by an unwind block.

R=clemensb@chromium.org

Bug: v8:8091
Change-Id: Ia149d2e81b1fbfa9209047b35ff0c9fedc1b8895
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2696662
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72785}