Show errors for KaTeX and mermaid on the preview tab (go-gitea#24009) (go-gitea#24019)

Backport go-gitea#24009 by @silverwind

There is a conflicting fomantic rule that hid the error messages inside
the markdown preview tab for things like mermaid or katex. Overruled it
to always show these errors.

<img width="774" alt="image"
src="https://user-images.githubusercontent.com/115237/230738528-322814c1-8994-495e-b901-bbb79b924ccb.png">

Co-authored-by: silverwind <me@silverwind.io>

Author: GiteaBot

web_src/css/markup/content.css

@@ -540,6 +540,7 @@
 }
 
 .markup-block-error {
+  display: block !important; /* override fomantic .ui.form .error.message {display: none} */
   border: 1px solid var(--color-error-border) !important;
   margin-bottom: 0 !important;
   border-bottom-left-radius: 0 !important;