thumbnailer: Always print errors from gs_file_read_noatime()

https://bugzilla.gnome.org/show_bug.cgi?id=699252
thumbnailer: Correctly check for errors

By checking the retval, not whether error was set.

https://bugzilla.gnome.org/show_bug.cgi?id=699252
thumbnailer: Check gdk_pixbuf_loader_close()'s retval

https://bugzilla.gnome.org/show_bug.cgi?id=699252