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!
It's due to sentinel configuration error, so I think it's better not to treat to connection failure.