public Status failed(Instant now, String message) {
            if (state != State.RUNNING)
                throw new IllegalStateException("Current state must be RUNNING when changing to FAILED");
            return new Status(startedAt, requireNonNull(now), progress, State.FAILED, requireNonNull(message));
        }
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