We often use raw assertPromiseResult with {success ==
assertUnreachable} for that. Having a separate helper increases
readability and allows us to generate consistent (and better) error
messages.

R=titzer@chromium.org

Bug: chromium:926311
Change-Id: I507941eacaafe6c576098d7829a76b27384a4fb6
Reviewed-on: https://chromium-review.googlesource.com/c/1456039Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59417}