code: def _start_callbacks(self, result, exception):
          
          if self._cancelled:
              raise CancelledError()
          if self.called:
              raise AlreadyCalledError()
  
          self._result = result
          self._exception = exception
          if self._exception:
              self._tb_info = ''.join(traceback.format_tb(sys.exc_info()[2])) 
          self.called = True
          self._do_callbacks()
docs: Perform the callback chain going back and forth between the callback
          and errback as needed.
  
          If an exception is raised and the entire chain is gone through without a valid
          errback then its simply logged.