content: Have errorCodeStyle ask for "Source Code Pro"

Like in the other places we ask for a monospace font.

(The exact choice of font isn't actually important here; the text is
just for displaying what message-content elements we haven't
implemented yet. But we're about to define a TextStyle constant to
encapsulate reasonable font-family fallbacks for a monospace font,
and it'll be convenient to use that constant here too. It'll use
"Source Code Pro" as its first choice.)

Author: chrisbobbe

lib/widgets/content.dart

@@ -639,4 +639,8 @@ InlineSpan _errorUnimplemented(UnimplementedNode node) {
 
 const errorStyle = TextStyle(fontWeight: FontWeight.bold, color: Colors.red);
 
-const errorCodeStyle = TextStyle(color: Colors.red, fontFamily: 'monospace');
+const errorCodeStyle = TextStyle(
+  color: Colors.red,
+  fontFamily: "Source Code Pro", // TODO supply font
+  fontFamilyFallback: ["monospace"],
+);