group-store: Disallow joining of nonexisting groups
group-store: Disallow joining of nonexisting groups

Returns %.n upon requesting a join of group that does not exist

ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit