Replace todo calls with error values in linux/platform (#8531)

We currently use a mix of unimplemented methods with empty bodies and
`todo!()` calls in linux/platform.

`todo!()`s cause crashes in runtime with accidental key presses or
clicks.

To avoid this, this PR replaces `todo!()`s in linux/platform with error
values.

This helps when working on Zed itself, testing PRs etc.

Release Notes:

- N/A