[rubygems/rubygems] Improve rake package test error message

https://github.com/rubygems/rubygems/commit/be962ca0c4

test/rubygems/test_rake_package.rb

@@ -8,7 +8,15 @@ class TestRakePackage < Minitest::Test
   def test_builds_ok
     output, status = Open3.capture2e("rake package")
 
-    assert_equal true, status.success?, "Expected `rake package` to work, but got errors: #{output}"
+    assert_equal true, status.success?, <<~MSG.chomp
+      Expected `rake package` to work, but got errors:
+
+      ```
+      #{output}
+      ```
+
+      If you have added or removed files, make sure you run `rake update_manifest` to update the `Manifest.txt` accordingly
+    MSG
   end
 
 end