better bundle renderer error handling

src/server/create-bundle-renderer.js

@@ -8,12 +8,16 @@ export function createBundleRendererCreator (createRenderer) {
       renderToString: (context, cb) => {
         runInVm(code, context).then(app => {
           renderer.renderToString(app, cb)
-        })
+        }).catch(cb)
       },
       renderToStream: (context) => {
         const res = new PassThrough()
         runInVm(code, context).then(app => {
           renderer.renderToStream(app).pipe(res)
+        }).catch(err => {
+          process.nextTick(() => {
+            res.emit('error', err)
+          })
         })
         return res
       }

src/server/run-in-vm.js

@@ -30,8 +30,6 @@ export default function runInVm (code, _context) {
   const res = Object.prototype.hasOwnProperty.call(m.exports, 'default')
     ? m.exports.default
     : m
-  if (typeof res.then !== 'function') {
-    throw new Error('SSR bundle should export a Promise.')
-  }
-  return res
+  // ensure returning a Promise
+  return Promise.resolve(res)
 }