ConsumerSupervisor does not restart killed child

[mmmries]

While experimenting with observer, my friend @newellista killed the process that the ConsumerSupervisor had started, and it didn't get restarted 😱

Instead the consumer supervisor just logged a message

15:43:58.924 [error] Elixir.Gnat.ConsumerSupervisor received unexpected message {:EXIT, #PID<0.361.0>, :killed}

So we need to make sure we are catching those exit messages in the ConsumerSupervisor and starting it back up.