If redis client throws exception, don't raise it

For throttling, when the redis client throws an exception, the request
ends up getting rate limited. Modify this to be similar to how
ActiveSupport.MemCacheStore functions (the read, write and increment
methods do not raise exceptions)
Merge pull request #49 from wkimeria/wip/dont_raise_redis_error

If redis client throws exception, don't raise it