Handle MemoryError in get_open_files()

get_open_files() is used by the AutoLocker to find files currently opended on the OS. We use it to lock/unlock documents on the server while DirectEditing.

But if the user does not have enough access rights to some file, the whole function will throw such error:

MemoryError: null
  File "nxdrive\autolocker.py", line 57, in _poll
  File "nxdrive\autolocker.py", line 70, in _process
  File "nxdrive\autolocker.py", line 124, in get_open_files
  File "__init__.py", line 1137, in open_files
  File "lib\site-packages\psutil\_pswindows.py", line 635, in wrapper
  File "lib\site-packages\psutil\_pswindows.py", line 905, in open_files

For information, the process in error is psutil.Process(pid=404, name='chrome.exe', started='18:45:29').

The fix is quite simple: suppress those errors.