public void addFailedBucket(BucketId superbucket, BucketId progress, String errorMsg) {
        BucketId existing = failedBuckets.put(superbucket, progress);
        if (existing != null) {
            throw new IllegalStateException(
                    "Attempting to add a superbucket to failed buckets that has already been added: "
                            + superbucket + ":" + progress);
        }
        if (firstErrorMsg == null) {
            firstErrorMsg = errorMsg;
        }
    }
Returns a string (base64) encoding of the serial form of this token
documentapi/src/main/java/com/yahoo/documentapi/ProgressToken.java