I think it would be great to have more detailled error messages.
More than “Messenger App couldn’t be loaded”
ex: “invalid sheet, ‘url’ is missing”
When you develop your app for the first time, it’s a good way to get on the tracks.
When you get back on your app a few weeks later and the way it works is not fresh in your might, that relieves a lot of frustration and helps to get back in the tracks.
Note that detailled messages should be limited to apps in ‘staging’ or ‘beta’ mode.