With this patch, we add a method with signature `Unwrap() error` to all
errors signaling an unexpected status code. All of them contain a value
of type `gophercloud.ErrUnexpectedStatusCode` that exposes, among other
useful values, the body of the response sent by the server.

To access the response body of a request that resulted in an unexpected
response code, try to unwrap the returned error to
`gophercloud.ErrUnexpectedStatusCode`:

```Go
pages, err := containers.List(client, nil).AllPages()
if err != nil {
	var responseCodeError gophercloud.ErrUnexpectedResponseCode
	if errors.As(err, &responseCodeError) {
		log.Printf("unexpected response code. Response body:\n---\n%s\n---", responseCodeError.Body)
	} else {
		log.Printf("unknown error")
	}
}
```