docs: def error(self, err=None):
          """Update the circuit breaker with an error event."""
          if self.state == 'half-open':
              self.test_fail_count = min(self.test_fail_count + 1, 16)
          self.errors.append(self.clock())
          if len(self.errors) > self.maxfail:
              time = self.clock() - self.errors.pop(0)
              if time < self.time_unit:
                  if time == 0:
                      time = 0.0001
                  self.log.debug('error rate: %f errors per second' % (
                          float(self.maxfail) / time))
                  self.open(err)