guide: show exporting create_exception! exceptions in a module

[birkenfeld]

This was asked about in Gitter; #805 seemed to suggest to them that it is not possible.

[davidhewitt]

Thank you for adding this!

[kngwyu]

Can we use these exceptions for catching error?
In #805, somehow an exception raised in #[pyfunction] and imported one are treated as different errors.

[birkenfeld]

This worked when I tried it:

try:
    rustmod.func()
except rustmod.CustomError:
    ...

where rustmod.func() raises the exception.