We recently started emitting an error because a module's validation
doesn't reach the end (due to lack of exports).
This contained a mistake that prevents many most other types
of errors from actually getting out.

Ensuring export errors (or not reaching then end) doesn't clobber
other errors.

TODO(bradnelson): Figure out a way to structure tests so this gets
caught.

BUG=v8:4203
R=gdeepti@chromium.org

Review-Url: https://codereview.chromium.org/2632593002
Cr-Commit-Position: refs/heads/master@{#42301}