Add group_access field to all xblocks

TNL-670
I don't think "invalid" is the same as "deleted" and wouldn't recommend "invalid". 
I would say either "deleted", if we're OK to talk to just the most likely use case, or I like Jim's suggestion although "that does not exist" is better for globalization than "non-existent": 
"This xblock refers to a content group configuration that does not exist."
Also, is there a simple recommended action that we can include in the error message, similar to the content experiments error message? i.e. a suggested action to help the user get out of the bad state.