JCBC-253: Fail fast when no replica is available during observe.

This changeset checks if really a replica is available, not just
theoretically enough nodes configured. When the current replica
index requested is -1, fail. This is mostly the case when a
failover is triggered (no replica available), but no rebalance
has been triggered yet. This leads to timing out observes.

Change-Id: I19310e2c3ce6f0409970cb150d371687e03467d6
Reviewed-on: http://review.couchbase.org/24867
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>