Please, implement std::error::Error for ursa::CryptoError

[Revertron]

I see that ursa::CryptoError doesn't implement trait std::error::Error, that prevents us from comfortable error processing.

Thanks in advance!

[brentzundel]

We plan to address error handling after doing a significant refactor. It may be that we use std::error::Error, but that will be determined once the project is in a more mature state.