Return `Result` in `EvalNamespace::lookup`

[optozorax]

Hi, thanks for the great crate!

I want to return a Result with my error type in the lookup function in the EvalNamespace object. For example, there can be recursion where a is dependent on value a, and I can tell this only when the lookup function is called. And I want to show in an error message that problem exactly in the recursion of user-defined formulas.

I believe this can be done by adding an Error associated type to EmptyNamespace and return something like Result<f64, Either<Self::Error, fasteval::Error>> from the lookup function.