Handle when a group used to exist but no longer does

When a group used to exist (or for some reason still exists
in the groups list) we should avoid failing when getting
members of that group in the running of watches function and
should instead handle that case specially and trigger all
watches (to notify those watches that everyone was removed).

Change-Id: I5484857e6ea58f1cdec5f9f8fac69a3571e053ed
Merge "Handle when a group used to exist but no longer does"