Return NO_BUCKET instead of disconnect

If you try to connect to memcached and don't end up in a bucket
you're currently just disconnected if you try to run operations.
A better solution is to return this error back to the client, so
that it may differenciate between network failures etc.

Change-Id: Id5893607e8a7d351ff04270758446812a929c02b
Reviewed-on: http://review.couchbase.org/41852
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>