We were already handling the case that a called import throws, but if
it returned an error which is not convertible to a number, we failed
with a CHECK error.
This CL fixes this.

R=titzer@chromium.org

Bug: chromium:771970
Change-Id: I6c9983459109d49c43304610b696d49de986a250
Reviewed-on: https://chromium-review.googlesource.com/735354
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48912}