add support for resolveManyEntities to return a list of entities and errors

When using `resolveEntity`, then `resolveEntityGroup` populates the response with either an entity or an error, depending on what the resolver returns.

With `resolveManyEntities`, the plugin takes either the list of entities, or the errors; it would be nice to be able to return a mix of entities AND errors, like the end user would get in case `resolveEntity` is being used.

If you don't want to support it out-of-the-box, it would be nice if you could provide an option to override the federation template, then we would be able to customize it to our use case.