getConversationsFor() chokes on ThingError

Summary: Purely an internal error, but it's clogging up the logs

If you can't read a Thing, you can't see the Conversations -- this is enforced in ThingConversationsCore.receive(GetConversations), which returns a ThingError. But that ThingError isn't handled in ConversationFunctionsImpl.getConversationsFor(), which instead throws an class-cast exception. It should handle that more clearly and silently.