Optimize log information

[JiaRG]

RelationsTab/Source/ForceDirectedGraph/FactionNode.cs

Lines 15 to 17 in 30938fc

	if (faction.Leader() == null) {
	Log.Warning($"Faction {faction.GetFactionLabel()} has no leader, and will not be shown in faction view!");
	}

Use Log.WarningOnce(); to reduce repeated prompts