The extension tells you when a page couldn't be saved

After you select “Add to Screenly” or “Update Asset”, the pop-up waits until Screenly has accepted the page before it shows “Web page saved”.

That usually takes a moment. If it takes more than 2 seconds, the pop-up says “This is taking longer than expected” while it keeps waiting. If Screenly still hasn’t accepted the page after about 30 seconds, or saving fails, the pop-up shows “Failed to save web page”, so you know to try again.