fix(rustup-mode): return ExitCode(1) when update() fails

[rami3l]

Closes#3476, verified via local testing.

[djc]

This seems okay. Personally I feel like use of the custom ExitCode is a bad abstraction. One idea I've played with is that we should change types to be Result<T, Option<E>>, where the Some represents an error that should be yielded by the top-level caller while None means the error has already been reported. (We then report ExitCode::FAILURE if the Result is Err.)

[rami3l]

This seems okay. Personally I feel like use of the custom ExitCode is a bad abstraction. One idea I've played with is that we should change types to be Result<T, Option<E>>, where the Some represents an error that should be yielded by the top-level caller while None means the error has already been reported. (We then report ExitCode::FAILURE if the Result is Err.)

@djc Indeed, in other projects that I've worked with my principle is that we should never expose the exit code with the single exception of the main module, where some thiserror is converted to ExitStatus via an impl Termination.

But that kind of change is clearly out of the scope of this PR...