Ignore the error type in {assertThrows} only if it was not passed as an
argument. If users do not care about the error type they can user the
generic type {Error}. Before this change, an undefined error type would
simply be ignored. A simple typo could therefore disable the error type
assertion without being recognized.

Change-Id: I9becfd0bf14dcaa511854e65ff94f94481cc79b0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1585855
Commit-Queue: Frederik Gossen <frgossen@google.com>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61105}