Previous error message when trying to delete channel with active
participants in call:
<img width="2880" height="1800" alt="image"
src="https://github.com/user-attachments/assets/b11a0d75-438d-468f-8fe4-e0c249dd096c"
/>

After change (tested locally)

<img width="474" height="304" alt="image"
src="https://github.com/user-attachments/assets/1e887411-a851-4dc8-83dc-881f690c0ad9"
/>



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

Closes #ISSUE
https://github.com/zed-industries/zed/issues/53572

Release Notes:

- N/A or Added/Fixed/Improved ...
Added a clear error message upon trying to delete channel with active
participants