See https://github.com/tc39/proposal-weakrefs/issues/37

Drive-by: fix error messages.

BUG=v8:8179

Change-Id: I8608d09ec5a58c8b62eea4580be9415f6bb41586
Reviewed-on: https://chromium-review.googlesource.com/c/1319758
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57347}