message: log backtrace when logging last_seen_index error
diff: @@ -31,7 +31,7 @@ pub fn watch_registry(
  Ok(None) => {}
  Err(err) => {
  log::error!(
  - "queue locked because of invalid last_seen_index_reference in database: {}",
  + "queue locked because of invalid last_seen_index_reference in database: {:?}",
  err
  );
  build_queue.lock()?;
  