rust-pyo3: conversions to GraphError Python exception The nice thing is that with PyO3, exceptions can be instantiated without holding the GIL. Hence the only thing that prevents us to implement `Into<PyErr>` for `hg::GraphError` is that neither is defined by the current crate. We could use a wrapping "newtype", but the compiler is not so clever yet that it could chain automatically to two needed `into()`, so we'll end up with some type conversion anyway, involving something like `GraphErrorWrapper`. At this point, explicitly named methods are just simpler.