Allow SparkSubmit to throw exceptions instead of exiting / printing errors.

SparkSubmit now can be called by the SparkLauncher library (see SPARK-11035). But if the caller provides incorrect or inconsistent parameters to the app, SparkSubmit will print errors to the output and call System.exit, which is not very user friendly in this code path.

We should modify SparkSubmit to be more friendly when called this way, while still maintaining the old behavior when called from the command line.