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!
@airpingu My intention is separate two things which first one is "all sentinels doesn't monitor master named equals to parameter." and next one is "all sentinels are down!".
First one doesn't mean Redis instances are down, so I've assigned to JedisException.
Second one means Sentinel instances are down, so I've assigned to JediConnectionException.