Handle "Backend Error" appropriately

```shell
Internet connection present
Error during syncing occurred
`sync_down` took 4.505069971084595 seconds
Internet connection present
2020-01-23 14:59:09 - Refreshing access token
Error during syncing occurred
`sync_down` took 5.417584180831909 seconds
Internet connection present
Traceback (most recent call last):
File "sync_server.py", line 40, in
sync_note.sync_down(keep)
File "/Users/kkanev/projects/gkeep-sync/sync_note.py", line 21, in function_timer
value = func(*args, **kwargs)
File "/Users/kkanev/projects/gkeep-sync/sync_note.py", line 106, in sync_down
keep.sync()
File "/Users/kkanev/.local/share/virtualenvs/gkeep-sync-BXeQRjV-/lib/python3.7/site-packages/gkeepapi/__init__.py", line 850, in sync
changes = self._reminders_api.list()
File "/Users/kkanev/.local/share/virtualenvs/gkeep-sync-BXeQRjV-/lib/python3.7/site-packages/gkeepapi/__init__.py", line 458, in list
json=params
File "/Users/kkanev/.local/share/virtualenvs/gkeep-sync-BXeQRjV-/lib/python3.7/site-packages/gkeepapi/__init__.py", line 228, in send
raise exception.APIException(error['code'], error)
gkeepapi.exception.APIException: {'errors': [{'domain': 'global', 'reason': 'backendError', 'message': 'Backend Error'}], 'code': 503, 'message': 'Backend Error'}
```

Reproduce this "Backend Error" and find a solution that doesn't shout down the server.