Catch exceptions thrown by threads launched in ScriptOperator and get Shark to compile with latest Spark master.

Two additions in this PR, since both are relatively small and needed to get Jenkins passing.

- Exceptions thrown by threads launched in ScriptOperator are now caught and rethrown by on-complete-callbacks registered with the TaskContext used to compute the corresponding RDD.
- SparkHadoopWriter#createJobID() access was changed here: https://github.com/apache/incubator-spark/pull/88, but the fix is pretty small - just copied createJobID() and made it a nested function in FileSinkOperator...