Fix incorrect notification on update download failure

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

   E.g

   # 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.