Use 'failure' instead of 'error-chain'

[ark0f]

Also I updated examples

[Emulator000]

Thanks for this PR @ark0f! I'll review this shortly, I think that is more cleaner with failure but I don't know this crate yet, could you explain the major benefits? thanks!

[ark0f]

failure crate is more useful than error-chain:

• You don't need to write error-chain links or foreign links to other errors.
• You can write Result<T, failure::Error> instead of Result<T, std::io::Error>, Result<T, hyper::Error>, etc.
• You don't need to cast one error to another using map_err, because it is universal error type for all errors.

It is especially useful for futures, because one future can use just one error type.

[jtfell]

Is this likely to be merged soon @Emulator000? Use of failure is encouraged in the Rust ecosystem and allows easy integration with other crates (eg. diesel, actix-web)