Fix unsaved exception in backup/drivers

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

We should use the excutils.save_and_reraise_exception() in some cases.

Change-Id: I5d0ea53ba6c52138c71cca61aedbdf06338f2a7d
Closes-Bug: #1292380
Merge "Fix unsaved exception in backup/drivers"