public void addError(Error err) {
        if (reply != null) {
            reply.getTrace().swap(trace);
            reply.addError(err);
            reply.getTrace().swap(trace);
        } else {
            setError(err);
        }
    }
This is a convenience method to add an error to this. If a reply has already been set, this method will add the
error to it. If no reply is set, this method calls {@link #setError(Error)}. This method also fiddles with the
trace object so that the error gets written to it.

@param err The error to add.
messagebus/src/main/java/com/yahoo/messagebus/routing/RoutingNode.java