t_admin: catch any CouchbaseNetworkError (not just ConnectError)

Change-Id: I73efb1152436c36c9edba742109fad4ed8fc7902
Reviewed-on: http://review.couchbase.org/38813
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>