Cluster API: Improve feedback when things go wrong

The Cluster API traps stdout/stderr of controller and engines because it's meant to be used interactively. But that's where information about why connection failures occurred is going to be, and it's frustrating to need to start a whole new cluster with `log_level=logging.DEBUG` to get a run with output you can see.

It would be good to be able to get this info after a process died that you *didn't* expect to. The answer to this is probably to pipe process output to a *file* and keep track of that file and provide an API to retrieve it.