MB-8601: Calmer error message when zero vbuckets in map.

During moxi startup, moxi might be racing with cluster-mgr REST
availability and moxi might temporarily receive an empty vBucketMap.

Change-Id: I2c6f78dd75727ed55d7e0fe6e189a91ee890f5f0
Reviewed-on: http://review.couchbase.org/39897
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Steve Yen <steve.yen@gmail.com>
Reviewed-by: Bin Cui <bin.cui@gmail.com>