extensions: Fix a 500 error that can happen if a duplicate UUID
extensions: Fix a 500 error that can happen if a duplicate UUID

We weren't rolling back the transaction here, causing the decorator
to error out the site, as it should.

GNOME/extensions-web,GNOME/extensions-web,GNOME/extensions-web,magcius/sweettooth,GNOME/extensions-web,magcius/sweettooth