Fix incorrect notification on update download failure

Sid requested to merge sid/gnome-software:properly-handle-update-errors into main

This MR fixes an incorrect success notification when the download has failed.

To reproduce,

  1. Trigger a backend command which locks the backend, as shown below


    # apt-get dist-upgrade -V
    valac (0.56.12-1 => 0.56.13-1)
    valac-0.56-vapi (0.56.12-1 => 0.56.13-1)
    valac-bin (0.56.12-1 => 0.56.13-1)
    valadoc (0.56.12-1 => 0.56.13-1)
    56 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.
    Need to get 143 MB of archives.
    After this operation, 174 kB disk space will be freed.
    Do you want to continue? [Y/n] 
  2. Leave the above command in [Y/n] prompt

  3. Click on the "Download" button in GS.

  4. It display a "Software Updates Downloaded" notification, which is wrong.


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