Can't return Err(js_sys::Error)? in -> Result<(), JsValue> function

[cormacrelf]

Creating a js_sys::Error and converting it to a JsValue will erroneously cause the glue code to catch the error you're trying to create. (Bit weird, catch shouldn't catch an Error when it's just a return value from a new Error() call.) I just get this in the console, printed twice.

wasm-bindgen: imported JS function that was not marked as `catch` threw an error: message goes here

Stack:
__wbg_new_2e9dc2ca6bd84218@webpack-internal:///./pkg/index.js:107:21
__wbg_new_2e9dc2ca6bd84218@http://localhost:8080/index.js:68:66
js_sys::Error::new::h7c642be162d2b294@http://localhost:8080/15b9d87f0cb69b4970c6.module.wasm:wasm-function[90]:0x7b67
rust_webpack_template::tries_to_throw::hec9794da8aa9f021@http://localhost:8080/15b9d87f0cb69b4970c6.module.wasm:wasm-function[17]:0x2adb
tries_to_throw@http://localhost:8080/15b9d87f0cb69b4970c6.module.wasm:wasm-function[132]:0x8f2f
tries_to_throw@webpack-internal:///./pkg/index.js:27:53
@webpack-internal:///./js/index.js:2:7

Originally posted by @cormacrelf in #1742 (comment)

Repro:

use js_sys::Error;
use wasm_bindgen::prelude::*;
use web_sys::console;
#[wasm_bindgen]
pub extern "C" fn tries_to_throw() -> Result<(), JsValue> {
    console::log_1(&JsValue::from_str("we made it this far"));
    let _z = Err(Error::new("message goes here"))?;
    Ok(())
}

[Kirguir]

https://rustwasm.github.io/docs/wasm-bindgen/reference/attributes/on-js-imports/catch.html

[alexcrichton]

Thanks for splitting this out! I believe this is fixed by #1788 where we were erroneously logging errors as unexpected where they were actually expected.