LibWeb: Render HTML content if present for HTTP error pages

If an HTTP response fails with an error code (e.g 403) but still has
body content, we now render the content.

We only fall back to our own built-in error page if there's no body.

Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp

@@ -322,7 +322,7 @@ void ResourceLoader::load(LoadRequest& request, Function<void(ReadonlyBytes, Has
                     store_response_cookies(request.page().value(), request.url(), *set_cookie);
             }
 
-            if (!success || (status_code.has_value() && *status_code >= 400 && *status_code <= 599)) {
+            if (!success || (status_code.has_value() && *status_code >= 400 && *status_code <= 599 && payload.is_empty())) {
                 StringBuilder error_builder;
                 if (status_code.has_value())
                     error_builder.appendff("Load failed: {}", *status_code);