This makes the existing error message tests also test the error
produced by asynchronous compilation and instantiation.
It also slightly tweaks the error message to contain the name of the
API function invoked instead of "WebAssembly Instantiation".

R=titzer@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Bug: chromium:926311
Change-Id: If4ab963cee8267d43b289169d21b31637c471d6d
Reviewed-on: https://chromium-review.googlesource.com/c/1456085
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59442}