FIX: fails loud if default setting is not set

Noted:
- `push_api_secret_key` is set in initializer. Shimed with ''
- `default_theme_key` is set in seeding. Shimed with ''

lib/site_settings/yaml_loader.rb

@@ -13,11 +13,13 @@ class SiteSettings::YamlLoader
           # Get default value for the site setting:
           value = hash.delete('default')
           if value.is_a?(Hash)
-            raise Discourse::Deprecation, "Site setting per env is no longer supported. Error setting: #{setting_name}"
+            raise Discourse::Deprecation, "The site setting `#{setting_name}` can no longer be set based on Rails environment. See also `config/environments/<env>.rb`."
+          elsif value.nil?
+            raise StandardError, "The site setting `#{setting_name}` is missing default value."
           end
 
           if hash['hidden']&.is_a?(Hash)
-            raise Discourse::Deprecation, "Hidden site setting per env is no longer supported. Error setting: #{setting_name}"
+            raise Discourse::Deprecation, "The site setting `#{setting_name}`'s hidden property can no longer be set based on Rails environment. It can only be either `true` or `false`."
           end
 
           yield category, setting_name, value, hash.deep_symbolize_keys!