Replace assertErrorMessage in mjsunit test

Replaces assertErrorMessage by assertThrows. Previously
assertErrorMessage didn't assert the error message that was
provided.

Change-Id: I30410b43ff16db448776d9f3cae817b1c0966b3d
Reviewed-on: https://chromium-review.googlesource.com/c/1496973Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Sven Sauleau <ssauleau@igalia.com>
Cr-Commit-Position: refs/heads/master@{#59982}