Exception in getInitialDate returns HTTP 200
I was adding more tests for #132 and noticed that the current (`master`) behavior for unhandled exception is not really correct.

```vue
<template>
  <div>test</div>
</template>

<script>
export default {
  getInitialData ({ req }) {
    throw new Error(`oops`)
  }
}
</script>
```

will return HTTP 200:

```
❯ curl localhost:4000/error -v
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 4000 (#0)
> GET /error HTTP/1.1
> Host: localhost:4000
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 200 OK
< X-Powered-By: Express
< Date: Sat, 08 Dec 2018 08:20:53 GMT
< Connection: keep-alive
< Transfer-Encoding: chunked
<
Error: oops
    at getInitialData (error.vue:8:0)
    at _callee$ (app/server-entry.js:64:0)
```

I would expect it to return HTTP 500.