pub fn map_err<'a, I, O, A, E1, E2, F>( co: Coroutine<'a, I, O, Result<A, E1>>, f: F) -> Coroutine<'a, I, O, Result<A, E2>>where F: FnOnce(E1) -> E2 + Send + 'a,
Convert E1 into E2
Sugar over bind