* Add a snippet to visualise faulty stylesheets
As discussed in https://github.com/Jermolene/TiddlyWiki5/issues/6176, a faulty stylesheet can propagate and break several other stylesheets. This can be tricky to debug in a wiki with a lot of custom stylesheets.
This wikitext snippet will display a list of the stylesheets created/modified by the user and will display a red cross next to the one causing a parsing error.
* Improve the css code and add a message
This commit add a short message that provide guidance on how to fix the CSS error(s).
