[gnome-shell] Non-fast-forward update to branch wip/exalm/query-exists

[Alexander Mikhaylenko <alexm src gnome org>]

The branch 'wip/exalm/query-exists' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  63f3255... background: Check if the file exists before loading it

Commits added to the branch:

  5bb13f3... background: Handle errors when loading wallpapers