code: def list_failed_work_units(self, work_spec_name, start=0, limit=None):
          
          return self.registry.filter(WORK_UNITS_ + work_spec_name + _FAILED,
                                      start=start, limit=limit)
docs: Get a dictionary of failed work units for some work spec.
  
          The dictionary is from work unit name to work unit definiton.
          Only work units that have completed unsuccessfully are included.