private static Map<Integer, ContentNodeErrorStats> toEmptyErrorStats(Set<Integer> storageNodes) {
        Map<Integer, ContentNodeErrorStats> contentNodeErrorStats = new HashMap<>(storageNodes.size());
        for (Integer index : storageNodes) {
            contentNodeErrorStats.put(index, new ContentNodeErrorStats(index));
        }
        return contentNodeErrorStats;
    }
Stores, for each content node, the aggregate error reports from all distributors in
a content cluster, for a particular cluster state version (or attempt at reaching
a state version in the case the cluster does not converge). The per-node statistics
are sparse, so in a healthy cluster this is expected to be a lightweight mapping
from all nodes -> empty ContentNodeErrorStats instances.

@author vekterli
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/ContentClusterErrorStats.java