This patch alters the Python <-> executor protocol to pass on
exception data when they occur in user Python code.

Fix minor documentation formatting issues.

This should avoid exceptions caused by existing
files with different contents.

I also removed some unused code.

Passing large volumes of data through Py4J seems
to be slow.  It appears to be faster to write the
data to the local filesystem and read it back from
Python.

Objects serialized with JSON can be compared for equality, but JSON can be slow
to serialize and only supports a limited range of data types.