Return 404 quickly when asking for a topic that doesn't exist.

[ralphbean]

Sometimes, someone will query for a topic without a delta.. and that topic doesn't exist. Datagrepper does its best to look through the history at every message it has ever seen, but before it can respond with "no messages found" it times out.

We should do some kind of pre-flight check to see if the topic is even present so we can give people a timely 404.