Comment 7 for bug 1693315

Reviewed: https://review.openstack.org/493206
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=89b6da0c2832e739e6e0d30a0e205165005dc41c
Submitter: Zuul
Branch: stable/newton

commit 89b6da0c2832e739e6e0d30a0e205165005dc41c
Author: Matt Riedemann <email address hidden>
Date: Thu Jun 15 11:46:44 2017 -0400

    Provide original fault message when BFV fails

    When booting from volume and Nova is creating the volume,
    it can fail (timeout, invalid AZ in Cinder, etc) and the
    generic Exception handling in _prep_block_device will log
    the original exception trace but then raise a generic
    InvalidBDM exception, which is handled higher up and converted
    to a BuildAbortException, which is recorded as an instance
    fault, but the original error message is lost from the fault.

    It would be better to include the original exception message that
    triggered the failure so that goes into the fault for debug.

    For example, this is a difference of getting an error like this:

      BuildAbortException: Build of instance
      9484f5a7-3198-47ff-b728-178515a26277 aborted:
      Block Device Mapping is Invalid.

    To something more useful like this:

      BuildAbortException: Build of instance
      9484f5a7-3198-47ff-b728-178515a26277 aborted:
      Volume da947c97-66c6-4b7e-9ae6-54eb8128bb75 did not finish
      being created even after we waited 3 seconds or 2 attempts.
      And its status is error.

    Change-Id: I20a5e8e5e10dd505c1b24c208f919c6550e9d1a4
    Closes-Bug: #1693315
    (cherry picked from commit 20c4715a49a44c642882618f102cd0fc9342978d)
    (cherry picked from commit 0d5fd4356af38ca0487979ad614d294a2002911d)