- Add PyError class that wraps Python exception;
- change PyFunc::operator() return from void -> to error, which might be
  either PyError or special error indicating that Python interpreter is
  stopped.
- Provide way to reraise that Python exception.

We will need to see which exception, if any, PyFunc call raised in
sync.PyWorkGroup, where this python-level exception will need to be
propagated to PyWorkGroup.wait() and reraised from there.