message: Change the message in `expect`.
diff: @@ -635,9 +635,7 @@ impl<C: Context<ValidatorId = ValidatorId>, DS: Strategy<DeliverySchedule>>
  C::ValidatorId,
  &mut HashMap<C::ValidatorId, C::ValidatorSecret>,
  )| {
  - let v_sec = secrets
  - .remove(&vid)
  - .expect("Validator's secret key exists.");
  + let v_sec = secrets.remove(&vid).expect("Secret key should exist.");
  let (highway, effects) = {
  let highway_params: HighwayParams<C> = HighwayParams {
  