Improve error-reporting for issuegen

--HG--
extra : rebase_source : 78a85ba323abf7980bff56664d7f254e7c70f1cf
[css-cascade] Update DoC

--HG--
extra : rebase_source : 2d744ec5c427f32a3b57b58f346d568ca82178ca
[css-grid] Update example to new syntax for subgrids. We might want to consider improving the shorthand here...