Fix for missing ResourceNotFound error in fs.walk

fs/base.py

@@ -850,6 +850,10 @@ class FS(object):
         """
 
         path = normpath(path)
+
+        if not self.exists(path):
+            raise ResourceNotFoundError(path)
+
         def listdir(path, *args, **kwargs):
             if ignore_errors:
                 try: