SPARK-1572 Don't kill Executor if PythonRDD fails while computing parent

Previously, the behavior was that if the parent RDD threw any exception other than IOException or FileNotFoundException (which is quite possible for Hadoop input sources), the entire Executor would crash, because the default thread a uncaught exception handler calls System.exit().

This patch avoids two related issues:

  1. Always catch exceptions in this reader thread.
  2. Don't mask readerException when Python throws an EOFError
     after worker.shutdownOutput() is called.

Author: Aaron Davidson <aaron@databricks.com>

Closes #486 from aarondav/pyspark and squashes the following commits:

fbb11e9 [Aaron Davidson] Make sure FileNotFoundExceptions are handled same as before
b9acb3e [Aaron Davidson] SPARK-1572 Don't kill Executor if PythonRDD fails while computing parent