In the preparser we don't need to worry about adding duplicate error objects to
the list, so we can simply unconditionally set the error flag. Restructure the
accumulator so we can also guarantee that we're checking the same flag as we're
setting.

Change-Id: I6a22cae468e77e5c6283c6fe937ca655f73991ac
Reviewed-on: https://chromium-review.googlesource.com/c/1275813Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56561}