The name {kLastErrorMessage} is misleading, as it's not actually the
index of the last message (or reason), but one more (i.e. number of
messages / reasons). Thus this renaming.

R=mstarzinger@chromium.org

Bug: v8:7754
Change-Id: Id21edcecac84c0e6068423c6124ef2881116dc7c
Reviewed-on: https://chromium-review.googlesource.com/1145305
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54593}