magnum cmd should print more graceful error message

[Ma Wen Cheng]

run magnum command with unreasonable parameters, it will print some error tracback message to the screen which is not needed.
we can only print these message in debug mode.

ubuntu@ubuntu14:~$ magnum baymodel-show 111
ERROR: Baymodel 111 could not be found.
Traceback (most recent call last):

  File "/opt/stack/magnum/magnum/conductor/handlers/indirection_api.py", line 31, in _object_dispatch
    return getattr(target, method)(context, *args, **kwargs)

  File "/usr/local/lib/python2.7/dist-packages/oslo_versionedobjects/base.py", line 171, in wrapper
    result = fn(cls, context, *args, **kwargs)

  File "/opt/stack/magnum/magnum/objects/baymodel.py", line 125, in get_by_name
    db_baymodel = cls.dbapi.get_baymodel_by_name(context, name)

  File "/opt/stack/magnum/magnum/db/sqlalchemy/api.py", line 355, in get_baymodel_by_name
    raise exception.BayModelNotFound(baymodel=baymodel_name)

BayModelNotFound: Baymodel 111 could not be found.
 (HTTP 404)

it should be like the following, looks better
ubuntu@ubuntu14:~$ nova show 111
ERROR (CommandError): No server with a name or ID of '111' exists.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/237844

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/237844
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=a5df0e56b09dd152d2e6dc278c021b8831f4e41f
Submitter: Jenkins
Branch: master

commit a5df0e56b09dd152d2e6dc278c021b8831f4e41f
Author: wenchma <email address hidden>
Date: Wed Oct 21 09:41:29 2015 +0800

    Update devstack doc to cover the latest atomic image

    Change-Id: Ie142a190b9771ffe7d0512a6d49e5c56c90e0f69
    Closes-bug: #1507972