[SPARK-1840] SparkListenerBus prints out scary error message when terminated normally

Running SparkPi example gave this error.
```
Pi is roughly 3.14374
14/05/14 18:16:19 ERROR Utils: Uncaught exception in thread SparkListenerBus
scala.runtime.NonLocalReturnControl$mcV$sp
```
This is due to the catch-all in the SparkListenerBus, which logged control throwable used by scala system