Fix the behavior of swift-ring-builder list_parts before rebalance

The swift-ring-builder list_parts before rebalance failed abnormally so
this patch fix the behavior. After this patch applies the behavior is
completion normally with the following messages.

Specified builder file "<builder_file>" is not rebalanced yet.
Please rebalance first.

Closes-Bug: #1399529
Change-Id: I9e5db6da85de4188915c51bc401604733f0e1b77