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.

def list_failed_work_units(self, work_spec_name, start=0, limit=None):
        """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.

        """
        return self.registry.filter(WORK_UNITS_ + work_spec_name + _FAILED,
                                    start=start, limit=limit)