Missing slug in TabGroup gives confusing errors

[Radomir Dopieralski]

If the slug attribute is not defined on a TabGroup, the page is rendered properly, but switching the tabs doesn't work -- either giving confusing JavaScript errors about missing spinner, if preload = False, or just simply doing nothing, otherwise.

A NotImplementedError should be raised instead.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/78951

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/78951
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=ed3f76defd22bafe8604b44ffb3283cce96f58c5
Submitter: Jenkins
Branch: master

commit ed3f76defd22bafe8604b44ffb3283cce96f58c5
Author: Radomir Dopieralski <email address hidden>
Date: Fri Mar 7 07:28:48 2014 -0500

    Raise an error when no slug on tab group

    When a tab group has no slug defined, it behaves in very strange
    ways and the error is hard to find. This will make finding it much
    easier.

    Change-Id: I97c4cdc7a860e8c40ce17ee1a59384bbf85782ad
    Closes-bug: 1288650