Error when a handler returns something other than a Response

currently

```python
# @template('index.jinja')
async def index(request):
return {
'foo': 'bar'
}
```

will give cause an error in `aiohttp-debugtoolbar`:

```
Traceback (most recent call last):
File "/home/samuel/code/purge_cloudflare/env/lib/python3.5/site-packages/aiohttp/web_server.py", line 61, in handle_request
resp = yield from self._handler(request)
File "/home/samuel/code/purge_cloudflare/env/lib/python3.5/site-packages/aiohttp/web.py", line 249, in _handle
resp = yield from handler(request)
File "/home/samuel/code/purge_cloudflare/env/lib/python3.5/site-packages/aiohttp_debugtoolbar/middlewares.py", line 122, in toolbar_middleware
toolbar.status = response.status
AttributeError: 'dict' object has no attribute 'status'
```

instead I guess debug toolbar should show the
```
AssertionError: Handler should return response instance, got [middlewares []]
```

as it does for standard exceptions occurring in a handler.