Warn when CameraUI is despawned

[rileysu]

Objective

Fixes #4856

Solution

Warn when camera isn't present instead of using .expect()

[DJMcNab]

It's not clear to me that we should warn here. The fix used in #4527 follows the example of

bevy/crates/bevy_core_pipeline/src/main_pass_3d.rs

Lines 53 to 56 in d353fbc

	match self.query.get_manual(world, view_entity) {
	Ok(query) => query,
	Err(_) => return Ok(()), // No window
	};

We definitely should use an early return instead of a huge match block here, even if we do decide we want to warn here (i.e. use this PR).

[rileysu]

This seems to be handled in #4527