code: def errors(self):
          
          errors = []
          for meta in self:
              errors.extend(meta.errors)
          return errors
docs: get all the errors
  
          >>> gres = PlayMeta("operation")
          >>> res_plus = BasicPlayMeta(Composable(name="plus"))
          >>> gres.append(res_plus)
          >>> res_plus.add_error(ValueError("invalid data"))
          >>> res_moins = BasicPlayMeta(Composable(name="moins"))
          >>> gres.append(res_moins)
          >>> res_plus.add_error(RuntimeError("server not anwsering"))
          >>> gres.errors
          [ValueError('invalid data',), RuntimeError('server not anwsering',)]