Remove failure dependancy and favor std::error::Error

[TheLostLambda]

Hello,

I've recently come across this crate and am quite a fan! With that being said, during my use of the crate, I discovered that I couldn't use the ? operator in the context of a function returning Result<(), Box<dyn std::error::Error>>. This seems to have become a somewhat idiomatic way of dealing with errors in Rust.

The failure crate, while certainly having its upsides, has had its use in libraries discouraged (due to a lack of stabilization) and the standard library has been gradually incorporating fixes that the crate was intended to provide.

Finally, failure pulls in backtrace as a dependency, which breaks cross compilation from Linux -> macOS, which is what has brought me here. I'd suggest replacing the failure crate with the std::error implementation and would be more than happy to make a pull request.

Let me know what you think!