Environment cleanup/failover hook

[jamesbraza]

If a rollout is preempted (e.g. exceeds its timeout), it would be nice to let the environment clean itself up, or run some failover hook.

Currently with paper-qa, an on-timeout behavior is hacked into the runner: https://github.com/Future-House/paper-qa/blob/v5.20.0/paperqa/agents/main.py#L171-L178

It would be useful to:

1. Expose an on-exception hook in Environment
2. Bake calling this into LDP's runner code