fix CheckpointRDD getPreferredLocations java.io.FileNotFoundException
Throw exception if task result exceeds Akka frame size.

This partially addresses SPARK-747.
Abort job if result exceeds Akka frame size; add test.

Conflicts:
	core/src/test/scala/spark/DistributedSuite.scala
Fix SPARK-670: EC2 start command should require -i option.