Make error handling more convenient #114

It is often needed for application to perform a fallible operation in its handler. Currently one needs to manually handle failures. Maybe there's a way to make it convenient. Ideally, one would just ? on results, but we need to distinguish parsing errors from internal errors. Two approaches that come to my mind:

#[derive(Debug, Copy, Clone, Eq, PartialEq)]
enum HandlingErrorKind {
    Request,
    Internal,
    // Maybe more?
}

trait HandlingError: std::fmt::Display {
    fn error_kind(&self) -> HandlingErrorKind;
}

The user would then implement HandlingError for their error type and could just ? errors.

Another approach is to use just enum instead of a trait:

enum HandlingError<R: fmt::Display, I: fmt::Display> {
    Request(R),
    Internal(I),
}

This could be used easily like this:

let user_input = serde_json::from_slice::<UserInput>(request.body()).map_err(HandlingError::Request)?;
user_input.save_to_db().map_err(HandlingError::Internal)?;

What do you think?