rust-status: properly translate OSError to Python

This is probably never going to be called after the next few patches, but we
might as well make sure this is done correctly for the future rewrite.

Differential Revision: https://phab.mercurial-scm.org/D9319

rust/hg-cpython/src/dirstate/status.rs

@@ -10,6 +10,7 b''
 //! `rustext.dirstate.status`.
 
 use crate::{dirstate::DirstateMap, exceptions::FallbackError};
+use cpython::exc::OSError;
 use cpython::{
     exc::ValueError, ObjectProtocol, PyBytes, PyErr, PyList, PyObject,
     PyResult, PyTuple, Python, PythonObject, ToPyObject,
@@ -89,6 +90,7 b' fn handle_fallback(py: Python, err: Stat'
 
             PyErr::new::<FallbackError, _>(py, &as_string)
         }
+        StatusError::IO(e) => PyErr::new::<OSError, _>(py, e.to_string()),
         e => PyErr::new::<ValueError, _>(py, e.to_string()),
     }
 }