Comment 1 for bug 1116122

Ah, with regard to the less useful error message, my local settings had the following in it (the devstack default):

HORIZON_CONFIG = {
    'dashboards': ('project', 'admin', 'settings',),
    'default_dashboard': 'project',
}

which overwrites the much more useful HORIZON_CONFIG in settings.py, and gets merged with the Horizon default config at https://github.com/openstack/horizon/blob/master/horizon/conf/default.py which has empty arrays for the exception types that are surfaced back.

Removing or updating the local settings' HORIZON_CONFIG got me the nicer error message back.