fix: add error result type to preloadData

[eltigerchino]

fixes #12399

This PR changes the preloadData function so that it returns the 'error' type instead of 'loaded' when the page fails to load. It also returns the error that caused the loading to fail and the correct status (previously it was always 200).

EDIT: changed it to also return status for redirects

TODO:

• test
• changeset

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[PavelNuzhin]

Hello! Thank you for that change!

I'm currently testing changes and noticed that the error I throw from the page is always returned as an Internal Error (500) from preloadData(). However, it seems logical that I would see, for example, my error(401, "Test error") from the load() function.

This is because when SvelteKit encounters my error in load_route and passes it to handle_error, this check happens:

if (error instanceof HttpError) {
    return error.body;
}

The error I threw isn't recognized as an HttpError or a SvelteKitError. So the kit can't get its status and always returns Internal Error (500).

Here, for example, get_status function will never return my 401 status

export function get_status(error) {
	return error instanceof HttpError || error instanceof SvelteKitError ? error.status : 500;
}

Is it intended behavior?

Here`s whats happening in handle_error():

Actually i think only problem is that get_status and get_message return default values

[PavelNuzhin]

I fixed it for my project. It may be bad but it works for me :)
I really appreciate it if you could make some changes to the preloadData() error handling.

kit/src/utils/error.js

/**
 * @param {unknown} error
 * @returns {number}
 */
export function get_status(error) {
	if(error instanceof HttpError || error instanceof SvelteKitError)
		return error.status;

	if(error instanceof Object && 'status' in error && typeof error.status === 'number')
		return error.status;

	return 500;
}

/**
 * @param {unknown} error
 * @returns {string}
 */
export function get_message(error) {
	if(error instanceof SvelteKitError)
		return error.text;

	if(error instanceof Object && 'error' in error)
		error = error.error;
	
	if(error instanceof Object && 'message' in error && typeof error.message === 'string')
		return error.message;

	return 'Internal Error';
}