The extension tells you when a page couldn't be saved
After you select "Add to Screenly", the extension waits until Screenly has accepted the page, and shows "Failed to save web page" if it doesn't.
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.