Improve bootstrap checks error messages

When multiple bootstrap checks fail, it's not clear where one error
message begins and the next error message ends. This commit numbers the
bootstrap check error messages so they are easier to read.

Relates #24548

core/src/main/java/org/elasticsearch/bootstrap/BootstrapChecks.java

@@ -149,8 +149,10 @@ static void check(
 
         if (!errors.isEmpty()) {
             final List<String> messages = new ArrayList<>(1 + errors.size());
-            messages.add("bootstrap checks failed");
-            messages.addAll(errors);
+            messages.add("[" + errors.size() + "] bootstrap checks failed");
+            for (int i = 0; i < errors.size(); i++) {
+                messages.add("[" + (i + 1) + "]: " + errors.get(i));
+            }
             final NodeValidationException ne = new NodeValidationException(String.join("\n", messages));
             errors.stream().map(IllegalStateException::new).forEach(ne::addSuppressed);
             throw ne;