Return 404 when bucket is not found

The 'Admin by default' behaviour hid this issue, because for
administrator authentication is always successful

Change-Id: I294e8f294b3d043933301eee5d2cf11634c93bf9
Reviewed-on: http://review.couchbase.org/28747
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>