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!
Why not throwing JedisConnectionException as well? The failure of connecting to the Resis server should be considered a type of connection exception. Right?