Add an `error` listener to the Redis client.

This is probably the cause of `ETIMEDOUT` errors crashing cb_server. When the Redis connection fails for any reason (Redis being restarted, etc.), the node_redis client throws an exception instead of reconnecting unless there is an `error` handler registered on the client object.

Relevant code in node_redis: https://github.com/mranney/node_redis/blob/master/index.js#L196-200
Merge pull request #17 from sourcedelica/ggrossman/add_redis_error_handler

Add an `error` listener to the Redis client.