[SPARK-1853] Show Streaming application code context (file, line number) in Spark Stages UI

This is a refactored version of the original PR https://github.com/apache/spark/pull/1723 my @mubarak

Please take a look @andrewor14, @mubarak

These changes are just a refactoring of the code (from `case Some` and `case None` to `Option.orElse`), with no change in the logic.