There are other things to export beside functions. Thus, also print the
export kind when printing an error for duplicate export names.

R=titzer@chromium.org

Change-Id: I7477040dda274a16cfd776d7ac8db6e50a197b97
Reviewed-on: https://chromium-review.googlesource.com/564940Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46503}