rquickjs::Error::Exception should preserve the original value

[RReverser]

In JavaScript, while uncommon, it's possible to throw arbitrary values, not just Error objects and some libraries actually make use of it.

Right now rquickjs doesn't allow to retrieve this non-Error value and fails with conversion errors instead, as the Exception variant can't represent anything but error objects:

Exception {
    message: StdString,
    file: StdString,
    line: i32,
    stack: StdString,
},

Example:

fn main() {
    let runtime = rquickjs::Runtime::new().unwrap();
    let context = rquickjs::Context::full(&runtime).unwrap();
    context.with(|ctx| ctx.eval::<(), _>("throw 42;")).unwrap();
}

results in the following conversion error, with no way to retrieve the original error value:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: FromJs { from: "int", to: "object", message: None }', /home/rreverser/.cargo/git/checkouts/rquickjs-da202ee7abfef64e/29c5454/core/src/result.rs:478:41
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

For compatibility with arbitrary JS code, it should be possible to retrieve the original rquickjs::Value from the exception. As far as I can tell, QuickJS provides this functionality, and the conversion is imposed only at the rquickjs level.

[holly-hacker]

To be clear: this is a panic inside rquickjs and not in the user code.

[RReverser]

No, this is a panic in my own example code, where I'm unwrapping the result.

[RReverser]

Oh I see what you mean; yeah good point. I thought it's in my unwrap, but this makes it even worse.

[DelSkayn]

I have introduced a PR to fix this issue #148.

This will be a significant change to how javascript errors are handled in rquickjs but will allow you to retrieve any thrown javascript value.