QVariant: Print a warning when deserialized user type is unknown

The deserialized user type is now shown to the user to figure which
QMetaType registration is missing.

Change-Id: I4b7624827e479b1bea67065ce3542183b7355165
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>

src/corelib/kernel/qvariant.cpp

@@ -2070,6 +2070,7 @@ void QVariant::load(QDataStream &s)
         typeId = QMetaType::type(name.constData());
         if (typeId == QMetaType::UnknownType) {
             s.setStatus(QDataStream::ReadCorruptData);
+            qWarning("QVariant::load: unknown user type with name %s.", name.constData());
             return;
         }
     }