Use thiserror instead of failure

[tekjar]

thiserror implements std::error::Error so that users don't have to depend on failure crate externally or this library doesn't have to re-export failure crate.

Another reason is thiserror has become a defacto crate for error handling in rust and hence might provide better interoperability with other crates

I'll send a PR if this sounds good to you

[escwdev]

Sure that would be great @tekjar.

I've actually switched to thiserror on some other partial wrappers that I've written for other exchanges more recently I just haven't had time to prioritize changing it here.