For async instantiation of WebAssembly code we had the assumption that
a pending exceptions (an exception which comes from
execution JS code) and an ErrorThrower error cannot occur at the same
time. This assumption turned out to be wrong. With this CL we handle
this case by prefering pending_exceptions over ErrorThrower errors.

In addition I extended the tests for failing instantiation to also
exercise async instantiation, and I added a regression test.

R=clemensh@chromium.org

Bug: chromium:870646
Change-Id: I4cb54ff8642ad4ea193b20f79905c9f6508c2b2e
Reviewed-on: https://chromium-review.googlesource.com/1163511Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54940}