Extend the errors.js mjsunit test to also check for the message in the
generated errors. This will help catch bugs later, e.g. when
refactoring the way we output errors:
https://chromium-review.googlesource.com/c/565282

Drive-by 1: Fix a superfluous period in one error message.
Drive-by 2: Fix a weird exception catching construct in the test.

R=titzer@chromium.org

Change-Id: I1c2e92fb2c34a481cbf8802153f8502452d45348
Reviewed-on: https://chromium-review.googlesource.com/582960Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46858}