Introduce helper function for Python bindings to handle conversion of Rust error to Python `ValueError`

[derekpierre]

Suggestion from @piotr-roslaniec (#54 (comment)) :

   // Elsewhere
   fn map_py_value_err<T: fmt::Display>(err: T) -> PyErr {
     PyValueError::new_err(format!("{err}"))
   }

    <potential error returning call>.map_err(map_py_value_err)?;