Not having a .js handler is not an error

I don't have an application registered for .js files. When I run gnome-extensions create, it appears to do nothing:

$ gnome-extensions create --uuid=test --name=test --description=test
$

Actually, it has done exactly what it was supposed to do; it has created ~/.local/share/gnome-shell/extensions/test/extension.js . It just didn't tell me!

This change makes gnome-extensions create just print the name of the created file if the user doesn't have an application registered to open .js files.