implement Fail for all internal error types (even better: use err-derive)

[maackle]

https://rust-lang-nursery.github.io/failure/intro.html

Hooking into the Fail trait, we can unify all of our Result return values to be just Result<_, failure::Error>. There is also an impl From<std::error::Error> for Fail for free. This will make it a lot easier to make a lot more functions return Results where we wouldn't normally, like when locking a mutex: we can just mutex.lock()?.foo() instead of mutex.lock().unwrap().foo() without having to map_err or change the function return type.

[sphinxc0re]

The failure crate is already outdated. The future plan is to extend and thusly fix the Error trait of the stdlib. There are several examples of how we could harness this, for example: err-derive

[maackle]

@sphinxc0re so would it even be worth doing this, or should we wait for it in std?

[sphinxc0re]

I believe, the most sustainable way of moving error management into a declarative pattern would be using the err-derive crate. It moves with and not against or next to std