Previously, we weren't waiting on the status future early enough so we
would just hang if we weren't able to start the agent process.

I also added the recent stderr logs in there to help the user debug the
issue, since it is likely relevant in these cases.

<img width="902" height="226" alt="image"
src="https://github.com/user-attachments/assets/204e42ff-4c9b-49e7-8a6d-ecf7b022fbd0"
/>


Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- acp: Improve error messages if the ACP agent fails to start.