public void setError(Error err) {
        Reply reply = new EmptyReply();
        reply.getTrace().setLevel(trace.getLevel());
        reply.addError(err);
        setReply(reply);
    }
This is a convenience method to assign an {@link EmptyReply} containing a single error to this. This also fiddles
with the trace object so that the error gets written to it.

@param err The error to set.
@see #setReply(Reply)
messagebus/src/main/java/com/yahoo/messagebus/routing/RoutingNode.java