code: synchronized Map<String, List<String>> getTaskTrackerErrors() {
      // Clone the 'trackerToFailuresMap' and return the copy
      Map<String, List<String>> trackerErrors =
        new TreeMap<String, List<String>>(trackerToFailuresMap);
      return trackerErrors;
    }
docs: Get the information on tasktrackers and no. of errors which occurred
  on them for a given job.
  
  @return the map of tasktrackers and no. of errors which occurred
  on them for a given job.