fix: rework ReloadDeclarativeRawConfig error path

Now that ReloadDeclarativeRawConfig returns config bodies always, the
original error checking path didn't quite make sense. This extracts the
body read and error check from the status error check and removes the
response body from the status error.

kong/client.go

@@ -405,15 +405,12 @@ func (c *Client) ReloadDeclarativeRawConfig(
 	}
 	defer resp.Body.Close()
 
-	var b []byte
+	b, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("could not read /config %d status response body: %w", resp.StatusCode, err)
+	}
 	if resp.StatusCode < 200 || resp.StatusCode >= 400 {
-		b, err = io.ReadAll(resp.Body)
-		if err != nil {
-			return nil,
-				fmt.Errorf(`failed posting new config to /config: got status code %d
-				(and failed to read the response body): %w`,
-					resp.StatusCode, err)
-		}
+		return b, fmt.Errorf("failed posting new config to /config: got status code %d", resp.StatusCode)
 	}
 
 	return b, nil