Fail fast while initializing JedisSentinelPool

It's related to #726.

I modified JedisSentinelPool's behavior to fail fast on

* wrong master name is specified
* all sentinels are down or not reached

Actually I also think it should fail fast and let users know about it instead of keep retrying.

Please review and comment! Thanks!
I don't think you need to check if the master is null here. This logic is only called when constructing, which means the master must be null in the first place. After it's assigned, you will break the loop. Right?