"… It may be that it is perfectly possible to conclude that the Adjudicator was wrong on the merits. Such a conclusion might have justified overturning his decision under the earlier wider appellate jurisdiction (see this court's judgment in Subesh ) but the jurisdiction under Section 101 forbids in effect the IAT deciding the merits itself unless at least it first concludes that the Adjudicator's decision cannot stand because it is marred by error of law"