Allow observe and durability operations to fail in lcb_sched_fail()

Previously these commands would end up getting scheduled anyway

Change-Id: I11d6c8d457657622fbd4c8074494260567e73a37
Reviewed-on: http://review.couchbase.org/41612
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>