Improves `gpui_linux` error messages for missing feature flags.
Currently, you'd need to look at the source & Rust backtrace to diagnose
the error shown when the feature flags are missing.

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:

- Improve `gpui_linux` error messages for missing feature flags

---------

Co-authored-by: Tom Houlé <tom@tomhoule.com>