MB-11947 Use cached seqs during any error on get_seqs() call

Change-Id: I3baa5433d51b18663332d6e968093dd3a06c8564
Reviewed-on: http://review.couchbase.org/40618
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>