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].

Fixes: dd1f222f

[1] gnome-desktop@f9b2c480

#99