thumbnailer: Unbreak thumbnailing of GIFs
The GIF codec throws GDK_PIXBUF_ERROR_INCOMPLETE_ANIMATION if it's closed without decoding all the frames. The thumbnailer has always, both now with gdk_pixbuf_new_from_file_at_scale and earlier, decoded only the first animation frame, which is why this specific error needs to be ignored [1].
Fallout from dd1f222f