Define a library wide error type

[Nuttymoon]

With the library growing, I think defining a library Wide error type becomes necessary. Returning String as error does not scale well, and is hard to decide what to do from an user point of view.

Having something like this would help people consuming the library's life incredibly better (it could also be useful for the CLI -> towards the best approach I was talking about before)

enum AshError {
    NetworkError(String),
    SubnetError {id: Id, msg: String},
    ...
}

Originally posted by @gboutry in #10 (review)