code: public synchronized void reportDiagnosticInfo(TaskAttemptID taskid, String info) throws IOException {
      TaskInProgress tip = tasks.get(taskid);
      if (tip != null) {
        tip.reportDiagnosticInfo(info);
      } else {
        LOG.warn("Error from unknown child task: "+taskid+". Ignored.");
      }
    }
docs: Called when the task dies before completion, and we want to report back
  diagnostic info