Handle invalid issues (#18111)

* Handle invalid issues

- When you hover over a issue reference, and the issue doesn't exist, it
will just hang on the loading animation.
- This patch fixes that by showing them the pop-up with a "Error
occured" message.

* Add I18N

* refactor

* fix comment for lint

* fix unit test for i18n

* fix unit test for i18n

* add comments

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/base/head.tmpl

@@ -46,10 +46,13 @@
 			]).values()),
 			{{end}}
 			mermaidMaxSourceCharacters: {{MermaidMaxSourceCharacters}},
+			{{/* this global i18n object should only contain gereral texts. for specalized texts, it should be provied inside the related modules by: (1) API response (2) HTML data-attribute (3) PageData */}}
 			i18n: {
 				copy_success: '{{.i18n.Tr "copy_success"}}',
 				copy_error: '{{.i18n.Tr "copy_error"}}',
-			}
+				error_occurred: '{{.i18n.Tr "error.occurred"}}',
+				network_error: '{{.i18n.Tr "error.network_error"}}',
+			},
 		};
 		{{/* in case some pages don't render the pageData, we make sure it is an object to prevent null access */}}
 		window.config.pageData = window.config.pageData || {};