thumbnailer: Avoid double-setting errors
thumbnailer: Avoid double-setting errors

When the image loading failed already, don't capture the error from
gdk_pixbuf_loader_close() to avoid a run-time warning.

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf