CBQE-1038: exception handling for bucket/stats

when bucket stats timeout returning non-json
handle exception to prevent postcondition_handler
from failing (output will be seen in logs/celery-stats.log)

Change-Id: I7a24266e175901e1973e10772607a300cec5690e
Reviewed-on: http://review.couchbase.org/24548
Tested-by: Tommie McAfee <tommie@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>