Comment 6 for bug 1693315

Reviewed: https://review.openstack.org/493141
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=50eb1c6df4ef067e2f3de1df4e89dfab5df66c47
Submitter: Zuul
Branch: stable/ocata

commit 50eb1c6df4ef067e2f3de1df4e89dfab5df66c47
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.

    Conflicts:
          nova/compute/manager.py
          nova/tests/unit/compute/test_compute_mgr.py

    NOTE(mriedem): Conflicts in manager.py were just due to no longer
    using _LE in master (pike). The test conflict was just some tests
    added to pike which aren't in ocata in the same area of the file.

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