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
Merge "Raise an error when no slug on tab group"