RunCommand in postinst was confused by signal exit codes.

Update postinst RunCommand to more properly deal with exit codes based on
signals.

BUG=chromium:240738
TEST=trybot

Change-Id: Ie2d488950b70b7aa7a6e0a53b16bf6e0c2e09715
Previous-Reviewed-on: https://gerrit.chromium.org/gerrit/51351
(cherry picked from commit 9cfc5e39525afa506fe266e630c2414c5814d76d)
Reviewed-on: https://gerrit.chromium.org/gerrit/56132
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>