Refactor away from `anyhow` in `arti` crate

Back when arti was only an executable, it was reasonable to use anyhow to represent its errors. But now that we are exposing its functionality as a library, I think we should refactor to use error types in the style of our arti-client, rather than using anyhow.

I don't think it's important to do this everywhere; we can get away with only doing it for the APIs not marked as experimental-api. We can also skip doing this for main and main_main since we tell people not to use those.

Tentatively putting in %Arti 1.0.0: Ready for production use since it's an API break and it's not too much work.