message: Prevent from underflows.
diff: @@ -477,7 +477,7 @@ where
  0
  };
  let lowest_acceptable_era_id =
  - current_era + self.auction_delay - self.unbonding_delay;
  + (current_era + self.auction_delay).saturating_sub(self.unbonding_delay);
  let highest_acceptable_era_id = current_era + self.auction_delay;
  if era_id < lowest_acceptable_era_id || era_id > highest_acceptable_era_id {
  warn!(
  