print out folder id if no containing osd could be found

xtreemfs_client/dataDistribution.py

@@ -539,6 +539,8 @@ class DataDistribution(object):
                 one_osd.update_folder(folder, size)
                 found_containing_osd = True
                 break
+        if not found_containing_osd:
+            print("update_folder: could not find a containing OSD for folder id: " + str(folder))
         assert found_containing_osd is True
 
     def description(self):