CBQE-0: log.exception instead of log.fail

Change-Id: I819f2cba9e145f3976b653e8ca6a20ad790c6659
Reviewed-on: http://review.couchbase.org/26880
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>