Write failed task logs to separate log files

It would be nice if we could generate per-task log files for failed tasks, allowing us to easily see the full set of output for that task without having to scroll through potentially a huge amount of messages (especially if the task failed early and there were still dozens of others that ran).

An example of potential usage:

```
spread -fail-logs ./logs
```

could put logs of failed tests in a `./logs` directory.