Better errors

[vjoki]

Refactoring UnrarError seems necessary as there's a need for errors on the Rust side of things, and abusing the FFI error codes for that is quite problematic. There seems to be room for some other improvements as well.

Some ideas for changes:

• Move some of the error::Code logic to types within unrar_sys, for example RARReadHeader could return something like enum ReadResult {..} with the relevant Codes.
• The type parameter for the data type in UnrarError makes it kind of unwieldy, and should be removed.

Additionally it could be nice to have the error type(s) implement std::error::Error, either manually or by using something like thiserror. There's a nice survey on the current state of error handling in Rust, in case you're interested in using a library for this.