The app failed to download/update from the App Store

The installation of apps is fully under iOS control and when there are problems, they are on the App Store or iOS side. There is absolutely no way for apps to influence the installation process.

If the app installation fails, the best thing to try is to delete it and install it again from the App Store (and possibly restart the device before). You don't have to worry about the payment - App Store remembers you have already purchased it so you won't pay twice for it. You might also search the Internet if other users aren't experiencing the same problem - the App Store could be temporarily down, there may be some problem with the particular iOS version you are using etc.