Defer raising an exception when deleting volumes

When looping through block device mappings to delete volumes with
delete_on_terminate set the loop should not exit on an exception.
Instead the loop will continue and the last exception can optionally be
raised.

Change-Id: If1c6ec30c6f49009ef42f9a3b3df1594783b0ae5
Closes-Bug: #1341738
Merge "Defer raising an exception when deleting volumes"