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.