CBQE-997: print latest UI logs in case of rebalance failure

Change-Id: Ieb8f35646be1feea6e3fe396c3e123685b2524e0
Reviewed-on: http://review.couchbase.org/24201
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>