MB-100: print time in future.set_exception()

Change-Id: Icc1a7877cc53018b260ac679ab96d106c0ac5f3e
Reviewed-on: http://review.couchbase.org/24194
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>