Flusher fails to log: AttributeError: 'Response' object has no attribute 'exception'

`logtail/flusher.py` in step() at line 68

```python
            if response.status_code == 500 and getattr(response, "exception") != None:
                print('Failed to send logs to Better Stack after {} retries: {}'.format(len(RETRY_SCHEDULE), response.exception))
```