Error modules cleanup

[cknitt]

Currently we have both Exn and Error modules in the standard library, and we suggest to catch JS errors as Exn.Error(error) (with error having type exn).

Instead, we should document catching JsError(error) (built-in type) with error having type unknown and provide related helper functions in the Error module.

Possibly consider a different/better name than JsError. Error is not ideal either, as it conflicts with the Result module.