MB-8601: More helpful warning on moxi racy startup.

When packaged with Couchbase Server, the preeminent user of moxi
nowadays, there might be a service startup race where the moxi process
starts earlier than cluster-map REST service readiness.

Instead of emitting ERROR message, use softer WARNING verbiage
to try to enhance calm.

Change-Id: Id5485e83483c336ac22cb1ce187c7dffbcf4f0e1
Reviewed-on: http://review.couchbase.org/39895
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Steve Yen <steve.yen@gmail.com>