Fix connecting to a crashed runner while booting

This should fix #189 - Previously, the slave node logic would attempt to connect to the crashed process and trigger a crash of the zeus server itself; now, it detects crashes during these connections and sends a stack trace like you'd expect.

The thing I'm not sure about here is that this seems like a bit of a lazy fix (though the protocol seems correct, for what the surrounding code does). I feel like this the client handler is missing a state machine the lack of which we're trying to work around here. Guess rewriting that would be a bit of a bigger undertaking, though (-: