Add ERROR messages to help debugging Pixel/StripGeoModelXml

Adding some error messages to provide context for an issue that found while debugging.

NB @dzengerj @mamirono the "top level" ITkPixel.gmx file should contain somewhere a logicalVolume conforming to this naming in order to define the envelope containing the detector.

FYI @tadej @tlari @hhayward

InnerDetector/InDetDetDescr/PixelGeoModelXml/src/PixelDetectorFactory.cxx

@@ -121,15 +121,19 @@ namespace InDetDDSLHC {
     
     //
     unsigned int nChildren = world->getNChildVols();
+    bool foundVolume = false;
 
     for (int iChild = nChildren - 1; iChild>=0; --iChild) {
-      if (world->getNameOfChildVol(iChild) == "Pixel") {
+      if (world->getNameOfChildVol(iChild) == "ITkPixel") {
 	// The * converts from a ConstPVLink to a reference to a GeoVPhysVol;
 	// the & takes its address.
+	foundVolume  = true;
 	m_detectorManager->addTreeTop(&*world->getChildVol(iChild));
 	break;
       }
     }
+    
+    if(!foundVolume) ATH_MSG_ERROR("Could not find a logicalVolume named \"ITkPixel\" - this is required to provide the Envelope!");
 
     doNumerology();