Give user at least some idea on why rebalance can't be started.

Change-Id: Id56e6e9f6d72f02e51904d60767b587ab8339eae
Reviewed-on: http://review.couchbase.org/34272
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>