Handle not_a_leader error when forwarding whereis_name request.
Handle not_a_leader error when forwarding whereis_name request.

The end behavior is unchanged: 'undefined' is returned to the
caller. The difference is in the verbosity of logging. Without special
handling, for each failed attempt a crash report will be logged. Since
it's not entirely unusual for a short period of time to have a node
be mistaken about the leader, I decided to handle this specially.

Change-Id: I717f68fb43a3cdaf076c1e85cda92dee86b541e8
Reviewed-on: http://review.couchbase.org/89250
Tested-by: Aliaksey Artamonau <ce14b1c16e0fd654f926118eb8addbec5aff4ffa@couchbase.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server