Comment 2 for bug 1538837

Reviewed: https://review.openstack.org/243105
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=c824982e6a3d6660697e503f7236377cc8202d41
Submitter: Jenkins
Branch: master

commit c824982e6a3d6660697e503f7236377cc8202d41
Author: Javeme <email address hidden>
Date: Mon Nov 9 20:37:34 2015 +0800

    raise exception ComputeHostNotFound if host is not found

    When performing a live migration,I passed a wrong host parameter,
    and then I got a wrong result like this:
    "Compute service of host-1 is unavailable at this time".
    I thought the compute service stopped, but the fact was I passed
    a host id instead of a host name,therefore it could not find the host.

    In order to distinguish the error "host not found" from the error
    "service not available", I think we should raise a different exception
    ComputeHostNotFound instead of ComputeServiceUnavailable.

    This patch we will return more accurate error msg for live migration.

    Closes-Bug: #1538837
    Change-Id: I6ad377147070f85b9b1d5a1d1be459e890e02bcc