Show errors for KaTeX and mermaid on the preview tab (#24009)

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">

web_src/css/markup/content.css

@@ -541,6 +541,7 @@
 }
 
 .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;