Returns a future which yields just the valid listings.
This future never fails with an error, no matter what happens. Errors are completely
ignored. If nothing can be fetched, an empty collection is yielded.
If there is an error, but some results can be returned, this future will yield
those partial results. When using this future, it is a good idea to also check
the errors future so that errors can be displayed and handled.
Returns a future which yields just the errors which occurred.
If this future yields a non-empty collection, it is very likely that elements are
missing from the valid() set.
This future itself never fails with an error. In the event of an error, this future
will successfully yield a collection containing at least one exception.