Better error message for attach/detach interface failed

[Alex Xu]

Some time we can see attach/detach interface failed, but we didn't log the detail info, that's hard to debug.

for example:
http://logs.openstack.org/02/111802/1/gate/gate-tempest-dsvm-neutron/eff16a6/logs/screen-n-cpu.txt.gz?#_2014-09-24_07_54_12_206

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/123953

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/123953
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=336ffb6c446fedc1375b1570b1277055f12bb452
Submitter: Jenkins
Branch: master

commit 336ffb6c446fedc1375b1570b1277055f12bb452
Author: He Jie Xu <email address hidden>
Date: Thu Sep 25 14:46:01 2014 +0800

    Logging detail when attach interface failed

    When attach interface failed, there isn't any detail for
    error info. Change the log level to exception to get more detail.

    Change-Id: If531c32357dbb002a3e92e4ca559eeb7f6ab6d2d
    Closes-Bug: #1373761