ifErr method
Performs a side-effect if this is Err.
Implementation
@override
Resolvable<T> ifErr(
  @noFutures void Function(Sync<T> self, Err<T> err) noFutures,
) {
  return switch (value) {
    Ok() => this,
    Err<T> err => Sync(() {
      noFutures(this, err);
      return value;
    }).flatten(),
  };
}