This fixes a corner case which allowed warnings during the asm.js
instantiation to be promoted to actual exceptions. Even instantiation
attempts that fail are not allowed to throw exceptions observable by
JavaScript, but need to fall back to JavaScript execution.

R=clemensh@chromium.org
TEST=mjsunit/regress/regress-6203
BUG=v8:6203

Change-Id: I86f5a3adda4bcfe63b5cddc42d8ae1c3dbb88147
Reviewed-on: https://chromium-review.googlesource.com/468808
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44414}