DDG4: Fix output when getting exception in sensitive detector

DDG4/plugins/Geant4SDActions.cpp

@@ -97,7 +97,7 @@ namespace DD4hep {
 	    << " Post (" <<std::setw(24) << step->GetPostStepPoint()->GetMomentum() << ") "
 	    << std::endl;
 
-	std::cout << out;
+	std::cout << out.str();
 
 	return true;
       }