plugin-loader: handle offline update errors properly

Fix the issue that no notification after a failed offline update.

GNOME/gnome-software#666
2 jobs for failed-offline-update-notification in 2 minutes and 58 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #753347
debian-stable-x86_64

00:01:26

passed #753346
fedora-x86_64

00:02:58