retry method
Refetches the sources that are in error, and only those — an
OptionalQueryResult.optional source whose query failed included.
cancelRefetch as for refetch.
Implementation
Future<void> retry({bool cancelRefetch = true}) => Future.wait([
for (final source in _sources)
if ((_optionalOrigin[source] ?? source).isError)
source.refetch(cancelRefetch: cancelRefetch),
]);