sign_official_build: dump more info in resign_android_image_if_exists

We're getting silent errors in here somewhere.

BRANCH=none
BUG=chromium:1141907
TEST=none

Change-Id: I9af0a3ea1696920fe67c915660f82a68c1bddf34
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2504358
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Tested-by: LaMont Jones <lamontjones@chromium.org>