message: Propagate lock error.
diff: @@ -158,14 +158,14 @@ impl MempoolPrevalidator {
  // add NewHead to queue
  self.validator_event_sender
  .lock()
  - .unwrap()
  + .map_err(|e| format_err!("Failed to obtain the lock: {:?}", e))?
  .send(Event::NewHead(head.into(), block.header.clone()))?;
  }
  ShellChannelMsg::MempoolOperationReceived(operation) => {
  // add operation to queue for validation
  self.validator_event_sender
  .lock()
  - .unwrap()
  + .map_err(|e| format_err!("Failed to obtain the lock: {:?}", e))?
  .send(Event::ValidateOperation(
  operation.operation_hash.clone(),
  operation.operation_type,
  @@ -175,7 +175,7 @@ impl MempoolPrevalidator {
  ShellChannelMsg::ShuttingDown(_) => {
  self.validator_event_sender
  .lock()
  - .unwrap()
  + .map_err(|e| format_err!("Failed to obtain the lock: {:?}", e))?
  .send(Event::ShuttingDown)?;
  }
  _ => (),
  