redis-nonce-store should catch redis errors

[jasonblanchard]

Calling code may want to know when calls to @redis.get or @redis.set fail. For example, in this code, err should be handled when @redis.get fails, perhaps because the redis client can't connect to the redis instance. And then despite this comment, I think you actually want to know when that write failed - otherwise, you're vulnerable to replay attacks.

Additionally, this code currently treats all nonceStore errors from @nonceStore.isNew in _valid_oauth as an "Expired nonce," which may not be the case if it can error out for other reasons (i.e. a call to @redis.get failed). It looks like you already have a StoreError, so you could use that in RedisNonceStore.isNew and then switch on the error type in the @nonceStore.isNew callback to determine which error to send to the next callback and ultimately up to the calling code.

I'm happy to submit a PR for this, just want to make sure you guys are ok with the architectural ideas above. I guess, technically, it's kind of a breaking change if people are relying on "Expired nonce" even when its not?