Wrap exception hierarchy across modules

Hi everyone! I searched quite a bit on the documentation and elsewhere on the Internet, and I didn't find any example on how to wrap a hierarchy of C++ exceptions, including when they span several Python modules. There is the following example in test_exceptions.cpp (see PR #296), but it doesn't seem to be in the documentation, and doesn't span several modules:

auto ex5 = py::register_exception<MyException5>(m, "MyException5");
py::register_exception<MyException5_1>(m, "MyException5_1", ex5.ptr());

I wasn't quite sure what would be the best practice to get the base PyObject* exception when it is wrapped in another module. I ended up with the following which seems to work:

// In foo/module.cpp
PYBIND11_MODULE(foo, m) {
    py::register_exception<foo::Error>(m, "Error");
}

// In bar/module.cpp
PYBIND11_MODULE(bar, m) {
    py::object base = py::module::import("foo").attr("Error");
    py::register_exception<bar::Error>(m, "Error", base.ptr());
}

// Test
>>> import bar, inspect
>>> inspect.getmro(bar.Error)
(<class 'bar.Error'>, <class 'foo.Error'>, <class 'Exception'>, <class 'BaseException'>, <class 'object'>)

Question 1: Does this seems like best practice?

Question 2: If yes, would it make sense to add these two snippets in the documentation? (First example: how to wrap an exception deriving from another wrapped exception; Second example: how to do it when the base exception is wrapped in a different module). I'm happy to help with this and make a pull request if it makes sense.

Thanks!

Start by reviewing the existing hierarchy example in test_exceptions.cpp and the relevant exception-wrapping documentation. Confirm how register_exception uses a base exception from another module, then add the two requested examples if the approach is appropriate. Done means the documentation covers both same-module and cross-module exception hierarchies.