message: Return evidence if sender doesn't know about it.
diff: @@ -655,15 +655,25 @@ impl<I: NodeIdT, C: Context + 'static> HighwayProtocol<I, C> {
  vec![]
  }
  Some(observation) => match observation {
  - Observation::None | Observation::Faulty => {
  + Observation::None => {
  warn!(
  ?vid,
  our_next_seq,
  ?observation,
  - "`IndexPanorama` doesn't match `state.panorama` for validator"
  + "expected unit for validator but found none"
  );
  vec![]
  }
  + Observation::Faulty => {
  + let ev = match state.maybe_evidence(vid) {
  + Some(ev) => ev.clone(),
  + None => {
  + error!(?vid, instance_id=?self.highway.instance_id(), "panorama marked validator as faulty but no evidence was found");
  + return vec![];
  + }
  + };
  + vec![HighwayMessage::NewVertex(Vertex::Evidence(ev))]
  + }
  Observation::Correct(hash) => (their_next_seq..our_next_seq)
  .take(self.config.max_request_batch_size)
  .filter_map(|seq_num| {
  