blkdebug: show an error for invalid event names

It is easy to typo a blkdebug configuration and waste a lot of time
figuring out why no rules are matching.

Push the Error** down into add_rule() so we can report an error when the
event name is invalid.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
