On an error during {ProcessExports()}, we would just continue
execution, resulting in a DCHECK failure later.
I did not find any tests for exported globals, so I added a few
(including a regression test for the referenced bug).

R=ahaas@chromium.org
BUG=chromium:734295

Change-Id: I35370de934c274f870680c662ef848c72268a7bc
Reviewed-on: https://chromium-review.googlesource.com/539401
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46064}