MB-11533: Give general estimate if stream name doesn't exist

In the tap-vbtakeover command we give an estimate even if the
stream we are trying to get an estimate for doesn't exist. We
should do the same thing for upr-vbtakeover.

Change-Id: Ifa2061a26fab08f6d217bb9c486f93af46bf314a
Reviewed-on: http://review.couchbase.org/38840
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>