exception WallpaperUnreadable

That file is not a picture this compositor can put on the desktop.

Separate from SettingsWriteFailed because it is the opposite report. SettingsWriteFailed says the change happened and will not last; this says nothing happened at all — the old background is still up, unchanged and still saved. Folding the two into one exception would leave a settings app unable to tell the user which of those two very different things occurred.