refactor proposal, use enum's implementing std::error::Error for error reporting

[dvdsk]

Currently ziggurat uses static str slices containing a description of what went wrong. By instead building an error tree using enum's end users can easily match on errors and handle them. Even errors that can not be handled benefit if we implement std::error::Error. Then end users can print the cause of the error.

I suggest using the de-facto standard crate thiserror to derive std::error::Error on the enums.

Like #8 I would not mind implementing this.