controller: Use attempts when dialling hosts

See [this CI log](https://s3.amazonaws.com/flynn-ci-logs/flynn-build-20141206231324-a495e642-b9de254c05ca97d7c89e004750d14de13ddbd5d1-2014-12-06-23-21-15.txt) for an example of a panic due to the unhandled error.