Improve UX around "unauthorized" API exceptions.

Instead of blindly logging out the user when any API returns
a 401 or 403 response (which in most cases is due to a service
being down or misconfigured) we catch the error and inform the
user that they are not authorized for that data.

This is separate from being unauthorized for a dashboard or
panel in Horizon, since those are within our control and involve
security concerns around exposing admin functionalities to end
users. Those checks function as they have previously.

Fixes bug 1060426.

Change-Id: Ied800f10926ac5fb3b9ac1c1c26bbb4fa94a2557
Merge "Improve UX around "unauthorized" API exceptions."