code: def error(self):
          
          # Copy the error from any failed item to be the error for the whole
          # barrier. The first error seen "wins". Also handles the case where
          # the WorkItems passed into the barrier have already completed and
          # been marked with errors.
          for item in self:
              if isinstance(item, WorkItem) and item.error:
                  return item.error
          return None
docs: Returns the error for this barrier and all work items, if any.