[SPARK-1516]Throw exception in yarn client instead of run system.exit directly.

All the changes is in  the package of "org.apache.spark.deploy.yarn":
    1) Throw IllegalArgumentException in ClinetArguments  instead of exit directly.
    2) In Client's main method, if exception is caught, it will exit with code 1, otherwise exit with code 0.
    3) In YarnClientSchedulerBackend's start method, if IllegalArgumentException is caught, it will exit with code 1, otherwise throw that exception.

After the fix, if user integrate the spark yarn client into their applications, when the argument is wrong or the running is finished, the application won't be terminated.