Fix a crash when the engine database is removed

2018-12-10 18:08:19 2148 6472 DEBUG    nxdrive.osi        OS integration type: Windows
2018-12-10 18:08:19 2148 6472 DEBUG    nxdrive.options    Option 'log_level_file' updated: 'DEBUG' -> 'TRACE' [local]
2018-12-10 18:08:19 2148 6472 INFO     nxdrive.commandline Running on version 4.0.3
2018-12-10 18:08:20 2148 6472 INFO     nxdrive.engine.dao.utils Checking database integrity: 'C:\\Users\\Windows10\\.nuxeo-drive\\manager.db'
2018-12-10 18:08:20 2148 6472 INFO     nxdrive.manager    Proxy configuration is NoProxy<>
2018-12-10 18:08:20 2148 6472 ERROR    nxdrive.engine.engine Setting invalid credentials, reason is: found no password nor token in engine configuration
Traceback (most recent call last):
  File "E:\nxdrive\engine\engine.py", line 661, in stop
    if not self._local_watcher.get_thread().wait(5000):
AttributeError: 'Engine' object has no attribute '_local_watcher'