repl: Improve iopub connection error messages (#53014)

Coming from #51834, these would be more helpful than just that it
failed!

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:

- N/A
24a6008e5cMostlyK committed on 4/7/2026, 5:42:50 AM· committed by GitHubparent2aa1559
2 files changedLine totals unavailable