Plugin pre-exit hooks should be called when a task is cancelled

If a task is cancelled, sometimes plugin pre-exit hooks are not called. If a pre-exit hook cleans up docker state, or spins down containers, it might not happen causing bad state to persist.

Example log output:
```
# Received cancellation signal, interrupting
--
  | 🚨 Error: Error tearing down bootstrap: The plugin docker-control pre-exit hook exited with status -1
  | 2020-03-25 22:42:27 DEBUG Terminating bootstrap after cancellation with terminated
```

In this case the task exited with status of 255, but the agent was still available. Note that when the task exits with at status code of 1 it looks like the pre-exit hook is successfully called.