Ensure that get_tree_list() can actually browse a folder

PermissionError: [Errno 13] Permission denied: '/usr/sbin/authserver'
  File "nxdrive/gui/application.py", line 1445, in _handle_connection
    self._handle_nxdrive_url(url)
  File "nxdrive/gui/application.py", line 1387, in _handle_nxdrive_url
    func(path)
  File "nxdrive/gui/application.py", line 1552, in ctx_direct_transfer
    self.show_server_folders(engine, path)
  File "nxdrive/gui/application.py", line 781, in show_server_folders
    self.filters_dlg = FoldersDialog(self, engine, path)
  File "nxdrive/gui/folders_dialog.py", line 233, in __init__
    self._process_additionnal_local_paths([str(path)] if path else [])
  File "nxdrive/gui/folders_dialog.py", line 384, in _process_additionnal_local_paths
    for file_path, size in get_tree_list(path):
  File "nxdrive/utils.py", line 354, in get_tree_list
    yield from get_tree_list(Path(entry.path))
  File "nxdrive/utils.py", line 354, in get_tree_list
    yield from get_tree_list(Path(entry.path))
  File "nxdrive/utils.py", line 354, in get_tree_list
    yield from get_tree_list(Path(entry.path))
  File "nxdrive/utils.py", line 339, in get_tree_list
    with os.scandir(path) as it:

diff --git a/nxdrive/utils.py b/nxdrive/utils.py
index 0a48eb4b..9c2ef15c 100644
--- a/nxdrive/utils.py
+++ b/nxdrive/utils.py
@@ -320,9 +320,9 @@ def get_tree_list(path: Path) -> Generator[Tuple[Path, int], None, None]:
     Note: this function cannot be decorated with lru_cache().
     """
     try:
-        path.is_dir()
+        it = os.scandir(path)
     except OSError:
-        log.warning(f"Error calling is_dir() on {path!r}", exc_info=True)
+        log.warning(f"Cannot browse {path!r}", exc_info=True)
         return
 
     # Check that the path can be processed
@@ -336,7 +336,7 @@ def get_tree_list(path: Path) -> Generator[Tuple[Path, int], None, None]:
     yield path, 0
 
     # Then, yield its children
-    with os.scandir(path) as it:
+    with it:
         for entry in it:
             # Check the path can be processed
             if entry.name.startswith(Options.ignored_prefixes) or entry.name.endswith(