QmlDesigner: Improve error message for errors in QML when loading the document

Change-Id: I3c47c199291c3661d0efb3a6df36aa281438d199
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>

src/plugins/qmldesigner/designmodewidget.cpp

@@ -110,6 +110,8 @@ void DocumentWarningWidget::setError(const RewriterError &error)
         m_goToError->hide();
     }
 
+    str.prepend(tr("Cannot open this QML document because of an error in the QML file:\n\n"));
+
     m_errorMessage->setText(str);
     resize(layout()->totalSizeHint());
 }