fix: make error.message enumerable (#9440)

fixes #9010

---------

Co-authored-by: Rich Harris <git@rich-harris.dev>
Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>

.changeset/sharp-trainers-search.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: make `error.message` enumerable when sending `ssrLoadModule` error to client

packages/kit/src/exports/vite/dev/index.js

@@ -58,7 +58,16 @@ export async function dev(vite, vite_config, svelte_config) {
 			const msg = buildErrorMessage(err, [colors.red(`Internal server error: ${err.message}`)]);
 
 			vite.config.logger.error(msg, { error: err });
-			vite.ws.send({ type: 'error', err: err });
+			vite.ws.send({
+				type: 'error',
+				err: {
+					...err,
+					// these properties are non-enumerable and will
+					// not be serialized unless we explicitly include them
+					message: err.message,
+					stack: err.stack
+				}
+			});
 
 			throw err;
 		}