Raise an error if os.walk encounters a problem

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/reclass/storage/yaml_fs/directory.py b/reclass/storage/yaml_fs/directory.py
index 06479ce..718483d 100644
--- a/reclass/storage/yaml_fs/directory.py
+++ b/reclass/storage/yaml_fs/directory.py
@@ -40,9 +40,14 @@
 
     def walk(self, register_fn=None):
         if not callable(register_fn): register_fn = self._register_files
+
+        def _error(exc):
+            raise(exc)
+
         for dirpath, dirnames, filenames in os.walk(self._path,
-                                                      topdown=True,
-                                                      followlinks=True):
+                                                    topdown=True,
+                                                    onerror=_error,
+                                                    followlinks=True):
             vvv('RECURSE {0}, {1} files, {2} subdirectories'.format(
                 dirpath.replace(os.getcwd(), '.'), len(filenames), len(dirnames)))
             for d in dirnames: