dm verity: allow only one error handling mode

If more than one one handling mode is requested during DM verity table
load, the last requested mode will be used.

Change this to impose more strict checking so that the table load will
fail if more than one error handling mode is requested.

Signed-off-by: JeongHyeon Lee <jhs2.lee@samsung.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>