Use `Result<_, io::Error>` over `io::Result<_>`

Use `Result<_, io::Error>` over `io::Result<_>`

Personally I've always found the overload of a prelude enum to be
confusing, and never bothered to handle it properly in bindings as
a result. To avoid needing to do so now, we simply move the
newly-introduced `io::Result` usages over to
`Result<_, io::Error>`.