make STALE_REACTION a `StaleReactionError extends Error`

packages/svelte/src/internal/client/constants.js

@@ -28,7 +28,10 @@ export const LOADING_ATTR_SYMBOL = Symbol('');
 export const PROXY_PATH_SYMBOL = Symbol('proxy path');
 
 // allow users to ignore aborted signal errors if `reason.stale`
-export const STALE_REACTION = { stale: true };
+export const STALE_REACTION = new (class StaleReactionError extends Error {
+	name = 'StaleReactionError';
+	message = 'The reaction that called `getAbortSignal()` was re-run or destroyed';
+})();
 
 export const ELEMENT_NODE = 1;
 export const TEXT_NODE = 3;

packages/svelte/tests/runtime-runes/samples/get-abort-signal/_config.js

@@ -7,7 +7,11 @@ export default test({
 		await new Promise((f) => setTimeout(f, 50));
 
 		if (variant === 'hydrate') {
-			assert.deepEqual(logs, ['aborted', { stale: true }]);
+			assert.deepEqual(logs, [
+				'aborted',
+				'StaleReactionError',
+				'The reaction that called `getAbortSignal()` was re-run or destroyed'
+			]);
 		}
 
 		logs.length = 0;
@@ -21,6 +25,10 @@ export default test({
 		await new Promise((f) => setTimeout(f, 50));
 		assert.htmlEqual(target.innerHTML, '<button>increment</button><p>2</p>');
 
-		assert.deepEqual(logs, ['aborted', { stale: true }]);
+		assert.deepEqual(logs, [
+			'aborted',
+			'StaleReactionError',
+			'The reaction that called `getAbortSignal()` was re-run or destroyed'
+		]);
 	}
 });

packages/svelte/tests/runtime-runes/samples/get-abort-signal/main.svelte

@@ -11,7 +11,7 @@
 		await new Promise((f) => setTimeout(f));
 
 		if (signal.aborted) {
-			console.log('aborted', signal.reason);
+			console.log('aborted', signal.reason.name, signal.reason.message);
 		}
 
 		return c;