Make rusb-async::Error as public

[samueldotj]

I am using the Transfer::poll() and trying to differentiate between timeout and other errors. It is not possible today since the error enum is private.

The following line needs to change to pub?

rusb/rusb-async/src/lib.rs

Line 11 in 4d955e4

use error::{Error, Result};

Or the intention is not to expose Error?