Pass detailed error message to python when parsing an argument fails

[ricohageman]

This issue might be seen as a continuation of #1640, which introduced more elaborated error messages when parsing objects using the derived FromPyObject trait. However, the extended error messages are not properly passed to python when parsing arguments provided by python to a rust function fails.

Consider for example the following case. When passing a wrong type for the value field, the error message contains the fact that value cannot be parsed. However, it fails to explain why it failed to parse value. This is valuable information and thus I would like to get this information out.

Example rust code

#[derive(FromPyObject)]
struct ValueClass {
   value: usize,
}

#[pyfunction]
fn function_to_call_with_value_class(value: ValueClass) {
    // Do stuff with the argument.
}

#[pymodule]
fn module(_py: Python, m: &PyModule) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(function_to_call_with_value_class, m)?)?;
    Ok(())
}

Example python code

Class ValueClass:
    def __init__(self, value):
        self.value = value

# Should be okay
module.function_to_call_with_value_class(ValueClass(0))  

# Should tell that a string cannot be converted into a integer.
# Currently tells: argument 'struct_arg': failed to extract field ValueClass.value
module.function_to_call_with_value_class(ValueClass("unparsable string")) 

# Should tell that a negative integer cannot be converted into an unsigned integer.
# Currently tells: argument 'struct_arg': failed to extract field ValueClass.value
module.function_to_call_with_value_class(ValueClass(-5)) 

[ricohageman]

This is closed by #2178