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>

options/locale/locale_en-US.ini

@@ -104,10 +104,12 @@ error404 = The page you are trying to reach either <strong>does not exist</stron
 never = Never
 
 [error]
-occurred = An error has occurred
-report_message = If you are sure this is a Gitea bug, please search for issue on <a href="https://github.com/go-gitea/gitea/issues">GitHub</a> and open new issue if necessary.
+occurred = An error occurred
+report_message = If you are sure this is a Gitea bug, please search for issues on <a href="https://github.com/go-gitea/gitea/issues" target="_blank">GitHub</a> or open a new issue if necessary.
 missing_csrf = Bad Request: no CSRF token present
-invalid_csrf = Bad Request: Invalid CSRF token
+invalid_csrf = Bad Request: invalid CSRF token
+not_found = The target couldn't be found.
+network_error = Network error
 
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