Fix dropped exception for create_export in vol manager

If self.driver.create_export() fails, model_update is
False and this exception is therefore not logged,
causing things to break later in the execution path in
unexpected ways.

Closes-Bug: #1300148

Change-Id: I77600f3311efd71fa644adb6d65e4b19ab306203
Merge "Fix dropped exception for create_export in vol manager"