Start work on re-throwing exceptions across the language barrier.
Check Python exceptions are rethrown to PHP.
Register PyException properly. Add dummy exception info.
Move W_PyException to the right place.

The naming of py_wrappers.py and php_wrappers.py is really confusing.

Is a py_wrapper a PHP object wrapping a python object or a Python object
that wraps a PHP object. The name could mean either.

As it stands, py_wrappers means the outer wrapper is for python, thus
wrapping a PHP object, but I wish we could find better naming conventions.
Add an XXX to remind us to implement better exception tests.
Remove unneeded extends= in k_PyException class def.
Restructure PyException, add getMessage() test.
Remove inaccurate comment.
Pass PHP exceptions up to Python (basic support).
Revise some tests related to exceptions.
Add a failing tests for exceptoin chaining
Clean up PHPException related code.

A related diff in PyPy. Moves PHPException to the app-level.
Ensure exception chaining works.
Hook throwing of PHP exceptions from python into to_php()
Remove comments.
Sprinkle some JIT hints.