Core: Simplify MainWindow::init() signature

And adapt its only user.

Change-Id: Ifc1f6c7857951d6e1a68f5bc909146e9689c5428
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/mainwindow.cpp

@@ -288,13 +288,10 @@ MainWindow::~MainWindow()
     m_jsExpander = nullptr;
 }
 
-bool MainWindow::init(QString *errorMessage)
+void MainWindow::init()
 {
-    Q_UNUSED(errorMessage)
-
     m_progressManager->init(); // needs the status bar manager
     MessageManager::init();
-    return true;
 }
 
 void MainWindow::extensionsInitialized()