unregister panels best effort

modules/garr_horizon_dashboard_customization_module/garrcloud/override.py

 import horizon
+import horizon.base
 
 dashboard_slug = 'admin'
 panel_slugs = [
@@ -22,5 +23,9 @@ def unregister_panel(dashboard_slug, panel_slug):
     dashboard.unregister(panel.__class__)
 
 for panel_slug in panel_slugs:
-    unregister_panel(dashboard_slug, panel_slug)
+    try:
+        unregister_panel(dashboard_slug, panel_slug)
+    except horizon.base.NotRegistered: # ignore missing slugs
+        # TODO: log a meaningful message
+        pass