fix: correctly handle HttpError on the client (#8829)

Closes #7966

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/fuzzy-kangaroos-relate.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: correctly handle HttpErrors on the client side

packages/kit/src/runtime/client/client.js

@@ -745,7 +745,7 @@ export function create_client({ target }) {
 				server_data = await load_data(url, invalid_server_nodes);
 			} catch (error) {
 				return load_root_error_page({
-					status: 500,
+					status: error instanceof HttpError ? error.status : 500,
 					error: await handle_error(error, { url, params, route: { id: route.id } }),
 					url,
 					route
@@ -1693,7 +1693,8 @@ async function load_data(url, invalid) {
 
 	if (!res.ok) {
 		// error message is a JSON-stringified string which devalue can't handle at the top level
-		throw new Error(data);
+		// turn it into a HttpError to not call handleError on the client again (was already handled on the server)
+		throw new HttpError(res.status, data);
 	}
 
 	// revive devalue-flattened data

packages/kit/src/runtime/server/utils.js

@@ -2,7 +2,6 @@ import * as devalue from 'devalue';
 import { json, text } from '../../exports/index.js';
 import { coalesce_to_error } from '../../utils/error.js';
 import { negotiate } from '../../utils/http.js';
-import { has_data_suffix } from '../../utils/url.js';
 import { HttpError } from '../control.js';
 import { fix_stack_trace } from '../shared.js';
 
@@ -81,7 +80,7 @@ export async function handle_fatal_error(event, options, error) {
 		'text/html'
 	]);
 
-	if (has_data_suffix(new URL(event.request.url).pathname) || type === 'application/json') {
+	if (event.isDataRequest || type === 'application/json') {
 		return json(body, {
 			status
 		});