Implement std::error::Error

[schneems]

I want to use this in a Box<dyn Error> but the error trait was missing an implementation.

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

Errors may provide cause information. Error::source() is generally used when errors cross “abstraction boundaries”. If one module must report an error that is caused by an error from a lower-level module, it can allow accessing that error via Error::source(). This makes it possible for the high-level module to provide its own errors while also revealing some of the implementation for debugging.