fix(11.1-07): check go fences, README src= and shell fences from the AST

- goLang follows the highlighter's chroma lookup, so golang and main.go need src=
- a src= fence in a module README is refused and never captioned
- shell fences inside callouts and lists are command-checked
- a fence with four or more leading spaces is an indented code block

internal/docsite/docsite.go

@@ -262,7 +262,11 @@ func (s *site) checkContent() ([]Problem, error) {
 		return nil, err
 	}
 	problems = append(problems, fp...)
-	problems = append(problems, s.checkPolicy(docs)...)
+	pp, err := s.checkPolicy(docs)
+	if err != nil {
+		return nil, err
+	}
+	problems = append(problems, pp...)
 	return problems, nil
 }