Question: Since they have exoskeletons, how do they convey anger or rage?

You'd be able to tell by their voice, their glowing eyes, or their posture and other gestures. Wardens and the other Primordian races pretty much use the same physical communication humans do. While their jaw may move somewhat up or down for a smile or frown, it is very subtle and not something you can pick up from further away.
If you aren't used to a warden's company then excitedly telling them how great your day is going may be awkward, as they'll seemingly be looming over you expressionless lol.