gnome3.updateScript: fix tarball eval on nonexisting attrpaths

As reported in https://github.com/NixOS/nixpkgs/commit/974f11cb2972f470ab4b7a71fdf8f09dfdff562b#commitcomment-38735081,
the tarball will fail to evaluate when updateScript is given a non-existing attrPath because getAttrFromPath
uses abort, which terminates the evaluation.