In debug mode, always catch imported JS function's exceptions and log if they throw

[fitzgen]

When we are in debug mode, this

#[wasm_bindgen]
extern {
    fn foo() -> u32;
}

should emit JS glue code something like this:

const __wbg_import_function_foo_target = foo;
export __wbg_import_function_foo = function () {
    try {
        return foo();
    } catch (e) {
        console.error("wasm-bindgen: imported JS function that was not marked as `catch` threw an error:", e);
        throw e;
    }
};

This will help us debug when imported JS functions throw and we don't catch the errors.