unhandledRejectionHandler = (reason, pr) => {
    lastReasonId += 1;
    const reasonId = lastReasonId;

    // Update bookkeeping.
    mapSet(idToReason, reasonId, reason);
    weakmapSet(promiseToReasonId, pr, reasonId);
    finalizationRegistryRegister(promiseToReason, pr, reasonId, pr);
  }
Report all the unhandled rejections, now that we are abruptly terminating
the agent cluster.