The Rust-error-to-Exception conversion should include the Error::source chain

[jrobsonchase]

For example, see the original error in #61.

Instead of just stringifying the top-level error, we should also walk the chain of Error::sources and append them to the message.

For example:

let mut msg = format!("{e}");

let mut dyn_e = e.source();
while dyn_e.is_some() {
    write!(&mut msg, ": {}", dyn_e.unwrap()).unwrap();
    dyn_e = dyn_e.unwrap().source();
}