Fix response HTTP code with 404 when no such image

It will response HTTP code with 404 instead of 500 if change error message start with "No such".

Before: 
```
< HTTP/1.1 500 Internal Server Error
< Content-Type: text/plain; charset=utf-8
< Date: Mon, 15 Dec 2014 08:50:46 GMT
< Content-Length: 52
< 
Error: image dorowu/ubuntu-desktop-le-vnc not found
```

After:
```
< HTTP/1.1 404 Not Found
< Content-Type: text/plain; charset=utf-8
< Date: Mon, 15 Dec 2014 10:06:41 GMT
< Content-Length: 45
< 
No such image: dorowu/ubuntu-desktop-lxde-vn
```