Improve error message

Show more datails about what actually went wrong.

Change-Id: I418a4d1f433bd4d440fc34e9a4932a9ea010b174
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/qml/compiler/qqmlpropertyvalidator.cpp

@@ -276,7 +276,11 @@ QVector<QQmlCompileError> QQmlPropertyValidator::validateObject(int objectIndex,
                     }
                 } else {
                     if (!enginePrivate->propertyCacheForType(pd->propType())) {
-                        return recordError(binding->location, tr("Invalid grouped property access"));
+                        return recordError(binding->location,
+                                           tr("Invalid grouped property access: Property \"%1\" with type \"%2\", which is not a value type")
+                                           .arg(name)
+                                           .arg(QString::fromLatin1(QMetaType::typeName(pd->propType())))
+                                          );
                     }
                 }
             }

tests/auto/qml/qqmllanguage/data/invalidGroupedProperty.1.errors.txt

@@ -1 +1 @@
-5:5:Invalid grouped property access
+5:5:Invalid grouped property access: Property "o" with type "QVariant", which is not a value type

tests/auto/qml/qqmllanguage/data/invalidGroupedProperty.3.errors.txt

@@ -1 +1 @@
-4:5:Invalid grouped property access
+4:5:Invalid grouped property access: Property "customType" with type "MyCustomVariantType", which is not a value type