Custom error types 🥅

[doc-E-brown]

Replace anyhow with custom error types

anyhow::{Result, Error} has been used to get basic functionality working and to develop some of the fundamentals of the crate.
This is a temporary measure and custom error types are required to provide users of the API with more helpful information about the nature or types of errors.

Definition of Done
Implement and return custom error types from functions / methods.