@Override
        public String toString() {
            return state + (message != null ? " (" + message + ")" : "") +
                   ", started at " + startedAt +
                   (endedAt != null ? ", ended at " + endedAt : "") +
                   (progress != null ? ", with progress " + progress : "");
        }
Returns a copy of this with the given end instant and failure message, in state FAILED.
clustercontroller-reindexer/src/main/java/ai/vespa/reindexing/Reindexing.java