rust-pyo3: conversions to GraphError Python exception...
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.
