Implement `sources` on the type that derives Error

I think it'd be immensely useful if `iter_chain` can be added on the type that derives Error and delegated to std::error::Error's iter_chain on nightlies with `error_iter` feature. Sure, one can always convert it into the std::error trait object and do it from there, but I think this will be ergonomic to have it right on the type.

https://doc.rust-lang.org/std/error/trait.Error.html#method.iter_chain