bdb: Assign different codes for data validation errors

This patch adds BDB_ERROR_DATA_CHECK_SIG and BD_ERROR_DATA_SIGNED_SIZE
to distiniguish data signature validation errors.

'futility bdb --resign' uses these to decide whether to resign is needed
or not.

BUG=chromium:649554
BRANCH=none
TEST=make runtests

Change-Id: I19137801ece2424ae575092c51d02664c8b73ba3
Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/386795
Reviewed-by: Randall Spangler <rspangler@chromium.org>