Replace todo calls with error values in linux/platform

[osa1]

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

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[osa1]

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