It seems like calling `_get_state_group_for_events` for an event where the
state is unknown is an error. Accordingly, let's raise an exception rather than
silently returning an empty result.

When we are processing a `/backfill` request from a remote server, exclude any
outliers from consideration early on. We can't return outliers anyway (since we
don't know the state at the outlier), and filtering them out earlier means that
we won't attempt to calulate the state for them.