Don't hide error when failing to grab diag from node.

Change-Id: Ia9285c8349ea4ff32c3d884d9c68b0431fdb4f07
Reviewed-on: http://review.couchbase.org/33016
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>