This commit fixes a bug that the search_extension() function panics
when the "GET /store/_search" interface returns a 404 response.

```
GET /store/_search?query=<query string>
{"_id":"_search","result":"not_found"}
```

It also improves the panic message by including varaible "response" in it,
so that we can inspect the actual response.

```
let hits_json = response.remove("hits").unwrap_or_else(|| {
    panic!(
        "the JSON response should contain field [hits], response [{:?}]",
        response
    )
});
```