Fix rendering not ending after it failed

For instance, when input bindings would fail, the master renderer thread would terminate but the UI would still be in rendering mode with no way to truly abort the render.

src/appleseed.studio/mainwindow/rendering/renderingmanager.cpp

@@ -97,16 +97,20 @@ namespace
       private:
         MasterRenderer* m_master_renderer;
 
-        // The starting point for the thread.
+        // The entry point for the thread.
         void run() override
         {
+            RENDERER_LOG_DEBUG("master renderer thread has started.");
+
             set_current_thread_name("master_renderer");
 
             const MasterRenderer::RenderingResult rendering_result =
                 m_master_renderer->render();
 
-            if (rendering_result.m_status == MasterRenderer::RenderingResult::Failed)
+            if (rendering_result.m_status != MasterRenderer::RenderingResult::Succeeded)
                 emit signal_rendering_failed();
+
+            RENDERER_LOG_DEBUG("master renderer thread is ending...");
         }
     };
 }