handle async route errors

Related to sveltejs#487

templates/src/server/middleware/get_server_route_handler.ts

@@ -53,7 +53,12 @@ export function get_server_route_handler(routes: ServerRoute[]) {
 			};
 
 			try {
-				handle_method(req, res, handle_next);
+				const result = handle_method(req, res, handle_next);
+
+				// catch failures in async functions
+				if (Promise.resolve(result) === result) {
+					result.catch(handle_next);
+				}
 			} catch (err) {
 				handle_next(err);
 			}

test/apps/errors/src/routes/async-throw.json.js

@@ -0,0 +1,3 @@
+export async function get(req, res) {
+  throw new Error('oops');
+}

test/apps/errors/test.ts

@@ -112,6 +112,15 @@ describe('errors', function() {
 		);
 	});
 
+	it('does not serve error page for async non-page error', async () => {
+		await page.goto(`${base}/async-throw.json`);
+
+		assert.equal(
+			await page.evaluate(() => document.body.textContent),
+			'oops'
+		);
+	});
+
 	it('clears props.error on successful render', async () => {
 		await page.goto(`${base}/no-error`);
 		await start();