Rollup merge of #25302 - michaelsproul:error-markdown, r=pnkfelix

A few errors slipped through my filter. Markdown formatting is especially important now that http://doc.rust-lang.org/error-index.html is live!

Speaking of, the error index should probably be linked to from somewhere. It doesn't quite fit under any of the sections in the index, but I could create a new one for it? Or add it under "tools" despite it not exactly being an executable tool.