Implement std Error for decode and encode errors

[wezm]

This PR implements the std::error::Error trait for decode::Error and encode::Error. This makes these error types more versatile for some uses. E.g. it permits them to work with an std Error trait object (Box<dyn Error>).

Since the current Debug impl for decode::Error seemed to be more of a Display impl (require by std Errror) I changed it as such and added a derived Debug impl.

[AldaronLau]

@wezm Looks good! Thanks again.