mapErr<F extends Object> method
Transforms the error value with f if this is Err.
Result.err('bad').mapErr((e) => Exception(e)); // Err(Exception('bad'))
Implementation
Result<T, F> mapErr<F extends Object>(F Function(E error) f) {
if (isErr) return Result.err(f((this as Err<T, E>).error));
return Result.ok((this as Ok<T, E>).value);
}