Fix invalid credentials handling when polling for changes

2019-02-06 14:43:08 4387 139807082669824 ERROR    nxdrive.engine.workers Thread RemoteWatcher(139807082669824) exception
Traceback (most recent call last):
  File "site-packages/nuxeo/client.py", line 194, in request
    resp.raise_for_status()
  File "site-packages/requests/models.py", line 939, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 403 Client Error: Forbidden for url: https://intranet.nuxeo.com/nuxeo/site/automation/NuxeoDrive.GetChangeSummary

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "client/remote_client.py", line 116, in execute
    return self.operations.execute(**kwargs)
  File "site-packages/nuxeo/operations.py", line 199, in execute
    default=kwargs.get('default', object))
  File "site-packages/nuxeo/client.py", line 197, in request
    raise self._handle_error(exc)
nuxeo.exceptions.Unauthorized: Unauthorized(403), error: b'...', server trace: None

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "engine/watcher/remote_watcher.py", line 590, in _handle_changes
    self._update_remote_states()
  File "engine/activity.py", line 118, in func_wrapper
    func(*args, **kwargs)
  File "engine/watcher/remote_watcher.py", line 662, in _update_remote_states
    summary = self._get_changes()
  File "engine/watcher/remote_watcher.py", line 623, in _get_changes
    self._last_root_definitions, self._last_event_log_id
  File "client/remote_client.py", line 443, in get_changes
    lastSyncActiveRootDefinitions=last_root_definitions,
  File "client/remote_client.py", line 120, in execute
    raise Forbidden(stack)
nxdrive.exceptions.Forbidden: None