admin_t: Don't expect BadHandleError

The upcoming version of libcouchbase no longer throws this error. A
generic error is thrown instead.

Change-Id: I44571e2a86753f28521c3d19646e271dffce5365
Reviewed-on: http://review.couchbase.org/38221
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>