private static String toJson(Slime slime) {
        try {
            return Utf8.toString(SlimeUtils.toJsonBytes(slime));
        }
        catch (IOException e) {
            throw new UncheckedIOException(e);
        }
    }
Returns the "message" element contained in the JSON formatted response.
hosted-api/src/main/java/ai/vespa/hosted/api/ControllerHttpClient.java