rust-pyo3: defining GraphError

This pretty much parallels the way in works with `cpython`. A warning, though:
this new `pyo3_rustext.GraphError` is not the same as `rustext.GraphError`, yet
both subclass `ValueError`.

rust/hg-pyo3/src/exceptions.rs

@@ -0,0 +1,4
+use pyo3::create_exception;
+use pyo3::exceptions::PyValueError;
+
+create_exception!(pyo3_rustext, GraphError, PyValueError);

rust/hg-pyo3/src/lib.rs

@@ -1,6 +1,7
 use pyo3::prelude::*;
 
 mod dagops;
+mod exceptions;
 mod util;
 
 #[pymodule]
@@ -15,5 +16,6 fn pyo3_rustext(py: Python<'_>, m: &Boun
     let dotted_name = format!("mercurial.{}", name);
 
     m.add_submodule(&dagops::init_module(py, &dotted_name)?)?;
+    m.add("GraphError", py.get_type::<exceptions::GraphError>())?;
     Ok(())
 }