Graceful fallback for Redis failures

Graceful Redis Failover in src/config.php​

By default, if Redis is enabled in XenForo 2.x (natively or via Xon's SV\RedisCache add-on) and the Redis endpoint becomes unreachable, XenForo throws a fatal HTTP 500 error. Even a quick package upgrade or service restart will cause this. It annoyed me that I...