Tell user how to fix flatpak background permissions

If the user tries to back up automatically, but the permission
has been denied in the past, tell them how to fix it (going to
Settings application settings).

Unfortunately, we can't go further and open the settings panel,
because flatpak doesn't let us launch the control center.
3 jobs for master in 12 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #849996
flatpak
flatpak

00:11:44

passed #849994
reuse_lint

00:00:33

failed #849995
vala_lint

00:00:38

 
Name Stage Failure
failed
vala_lint Test
Restoring cache
Downloading artifacts
Running before_script and script
$ io.elementary.vala-lint -c vala-lint.conf .
.deja-dup/Background.vala
133.120 error Line exceeds limit of 120 characters (currently 146 characters) line-length
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1