MB-36770: Print message if no buckets found for cbstats -a call
MB-36770: Print message if no buckets found for cbstats -a call

Simply adds a short message clarifying that cbstats found no buckets to
perform the stats call on, rather than silently exiting.

Change-Id: Ie9846867d702b64d83a814ead0a80c0cd87bcf14
Reviewed-on: http://review.couchbase.org/117417
Tested-by: James Harrison <5c25a7b3db09830813688775bfc8e6de736a230c@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine