don't fail on finders without storages attribute

pipeline/storage.py

@@ -131,7 +131,12 @@ class BaseFinderStorage(PipelineStorage):
     def listdir(self, path):
         directories, files = [], []
         for finder in self.finders.get_finders():
-            for storage in finder.storages.values():
+            try:
+                storages = finder.storages.values()
+            except AttributeError:
+                continue
+            else:
+                for storage in storages:
                     try:
                         new_directories, new_files = storage.listdir(path)
                     except OSError: