Fix trivia bot can't find key

[showell]

When you load the trivia bot, you get an error like this:

zulip_bots.lib.StateHandlerError: Error fetching state: {'result': 'error', 'msg': 'Key does not exist.'}

I believe this is partly due to how our Storage class works. We need a way to check that the key exists or not without an exception being thrown.

[orientor]

@showell Is this issue still on?

[orientor]

@showell Trivia quiz bot seems to work fine for me.

[TAM360]

@orientor I can't reproduce this issue either. @showell Are we sure that issue hasn't been resolved in some previous commit?