Use `failure` crate for errors instead of a enum

[heartsucker]

[wellsie1116]

Are there any concerns with tuf::Error not being able to implement PartialEq/Eq? I'd like to extract some of the error cases currently stored in Error::Opaque into their own variants, but errors like failure::Error, anyhow::Error, hyper::Error and std::io::Error, for example, aren't PartialEq.

This can be a problem with assert_eq! in tests, but assert_matches! mostly solves that issue.