Reflect exit code in uv tool run and uv run

[charliermarsh]

Summary

Closes #6710.

[charliermarsh]

Apparently on Unix you get None if it exits with a signal.

[charliermarsh]

I don't really understand when this could fail.

[zanieb]

For some reason, I wrote this previously

uv/crates/uv/src/bin/uvx.rs

Lines 50 to 55 in fd408c4

	// Fail with 2 if the status cannot be cast to an exit code
	Ok(status) => u8::try_from(status.code().unwrap_or(2)).unwrap_or(2).into(),
	Err(err) => {
	eprintln!("error: {err}");
	ExitCode::from(2)
	}

[charliermarsh]

An alternative is that we use exec-style execution instead. Then we'd get exit codes, signals, etc. by default. The downside is that we can't do any of our own custom error-handling. So we'd lose these really nice error messages: