message: Don't treat the lack of the unit file as an error
diff: @@ -116,7 +116,11 @@ impl<C: Context> ActiveValidator<C> {
  }
  fn write_last_unit(&mut self, hash: C::Hash) -> Option<()> {
  - let unit_hash_file = self.unit_hash_file.as_ref()?;
  + let unit_hash_file = if let Some(file) = self.unit_hash_file.as_ref() {
  + file
  + } else {
  + return Some(());
  + };
  self.own_last_unit = Some(hash);
  let mut file = File::create(unit_hash_file).ok()?;
  let bytes = serde_json::to_vec(&hash).ok()?;
  