Fix folders watch on Windows

Issue

For an unknown reason, we can end on this error when unwatching a folder on Windows:

KeyError: WindowsPath('C:/Users/nuxeo/Documents/Nuxeo Drive 5')
 File "nxdrive\gui\api.py", line 557, in unbind_server
 File "nxdrive\manager.py", line 660, in unbind_engine
 File "nxdrive\osi\windows\windows.py", line 239, in unwatch_folder
 

Fix

In 2 parts:
1) Filter out this specifc error.
2) When registering watched folders, we should use the current folders in use instead of looking for ones in the registry.