message: panic if existing era is provided as a new era
diff: @@ -120,9 +120,7 @@ where
  secret_signing_key: SecretKey,
  ) -> Effects<Event<I>> {
  if self.active_eras.contains_key(&era_id) {
  - error!(?era_id, "era already exists");
  - // TODO - should panic?
  - return Effects::new();
  + panic!("{:?} already exists", era_id);
  }
  let sum_stakes: Motes = validators.iter().map(|(_, stake)| *stake).sum();
  @@ -146,7 +144,7 @@ where
  };
  let params = HighwayParams {
  - instance_id: hash(format!("test era {}", era_id.0)), // TODO: get a proper ID?
  + instance_id: hash(format!("Highway era {}", era_id.0)),
  validators: weights,
  };
  let public_key = PublicKey::from(&secret_signing_key);
  