When restoring a backup, if app is not installed, help the user install it

[paulproteus]

Steps to reproduce:

• Make sure you don't have Etherpad installed
• Upload this ZIP file Asheesh tests backups.zip

Expected behavior:

• Show the user something actionable, such as "This app isn't installed yet. Go check if it is on the app market and install it if possible."
• Or, check the "app index" (JSON cached from the app market) to see if we can show the app name and then if so provide an install link

Actual behavior:

• Confusing error message: “Upload failed: App id for uploaded grain not installed [500]”

Thanks to @html5cat for reporting the issue to me elsewhere.

[ocdtrekkie]

Definitely this. And if you aren't sure which app the grain was from to begin with, currently there's no way to find out! The biggest question is that the grains are package version dependent, I believe... does the app market store the older versions of package to offer up? If someone's restoring a backup, it may be because their upgraded grain was problematic, so assuming the newest version is acceptable may not be a good idea.