Add logging notice

Author: keelerm84

ldclient/impl/integrations/files/file_data_sourcev2.py

@@ -423,5 +423,6 @@ def _check_file_times(self):
             try:
                 ret[path] = os.path.getmtime(path)
             except Exception:
+                log.warning("Failed to get modification time for %s. Setting to None", path)
                 ret[path] = None
         return ret