Exn to JSError.t

Ah, a third option.

The underlying problem is that, within a Promise.make call, reject needs to be called on items of the same type. We can’t cast exn to JsError.t, but we can convert JsError.t to JSExn.t:

    k->Context.onError((error: JsError.t) => {
      k->Context.quit
      reject(JsError.toJsExn(error))
    })
    k->Context.onLoad(() => {
      testFn(k)
      ->thenResolve(resolve)
      ->catchJSError(err => reject(JsError.toJsExn(err)))
      ->finally(
        () => {
          k->Context.quit
        },
      )
      ->ignore
    })

Wrapping the errors into exceptions like this would then let us mix in exn via JsExn.fromException