hbm-minigbm: export and use hbm::Result<T>

In rust, it is perfectly fine (in fact, recommended by std::io docs) to reuse the Result name when defining a custom Result type in a crate, then allow external users to use it with full namespaced references (e.g. io::Result).

Let's export the existing custom hbm::Result to follow the convention and re-use it from hbm-minigbm.