shell: Ignore failure to hold the scheduler daemon

It’s not fatal, it just means that the daemon might go away while
gnome-software is running.

It returns `G_DBUS_ERROR_FAILED` if the client process (gnome-software)
already has a hold on the daemon, so ignore that completely.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Status Job ID Name Coverage
  Build
passed #733302
debian-stable-x86_64

00:01:22

passed #733301
fedora-x86_64

00:03:13