Use a custom Fail type instead of format_err!

[csssuf]

format_err! and string-based errors are good for this initial prototype, but aren't a great long-term solution for a library API. Instead, some type should implement Fail, and the library should use errors of this type. Since ReturnCode already exists and corresponds to the different error codes returned from libyubihsm, it would be a good candidate for implementing Fail.

[tarcieri]

As I noted on another issue, what failure enables which is pretty nice is having several different error types in a single crate, all of which compose as failure::Error in Result types.

For this crate, you probably want a ConnectorError and a SessionError type