Use int instead of byte to store the source position when computing a
location based on the stack trace stored in an error object.
Also add tests, since this code path was not covered before (not even
for small position where it would have succeeded).
Also, add some comments about which positions are 0-based and 1-based.

R=titzer@chromium.org

Change-Id: I313dcd6c47b77093ced9bb687415715d04eafb97
Reviewed-on: https://chromium-review.googlesource.com/645527Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47739}