JSCBC-151: Fix mock BucketManager not reporting error for missing ddocs.

getDesignDocument now throws a not_found error if the design document
does not exist.

Change-Id: Ibeb7055e44665a3c2968e7467f83012e94a46081
Reviewed-on: http://review.couchbase.org/41989
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Brett Lawson <brett19@gmail.com>