Make error handling more principled and future proof

[aljoscha]

The failure crate was deprecated and they seem to recommend using thiserror: https://github.com/rust-lang-nursery/failure.

We should also not blindly return just any error (i.e we shouldn't use anyhow) but think about the errors we return. We also need to make sure that we can extend the error classes in the future without breaking user code.