retry 503 errors on the LP directory service.

xmlrpc.launchpad.net will from time to time, naturally, give a 503. When this happens bzr should retry once or twice. (Even if the service is load balanced at the far end, when an error occurs, it will get relayed back to bzr because the load balancer won't buffer the content (to prevent DOS)). So wanting this isn't an indication of a flaky service, its an appropriate measure to take.