[docs] rust frontend crash when a command returns an error with current examples

With the current examples, when invoking a command that return an error (a Err variant of a Result type) from a rust frontend, the rust frontend panic with an error about "unexpected exception".

The solution I found is to adjust the wasm_bindgen definition to the following:

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(catch, js_namespace = ["window", "__TAURI__", "core"])]
    async fn invoke(cmd: &str, args: JsValue) -> Result<JsValue, JsValue>;
}

And adjust the code using invoke to handle the result.

Maybe a better idea is to define two invoke external functions, one with the catch and one without, I don't know.

Locate the current Rust frontend examples and their wasm_bindgen invoke definition, then reproduce a command that returns an Err result. Update the documented example so this case is handled without an unexpected-exception panic, and verify the example behavior.