Fix unsaved exception in store.rbd.Store.add()

When an exception occurs during exception handling, it lose the
information of the first exception.

We should use the excutils.save_and_reraise_exception() in this case.

Change-Id: I1154e17d98804270f9578bc34a5fca6629bd9af4
Closes-Bug: #1292455
Merge "Fix unsaved exception in store.rbd.Store.add()"