(follow up issue) HTML error page response for ZenTao

About

This was generated from this discussion !69602 (comment 705842813).

If a user encounters an error when viewing a ZenTao issue, they currently see a JSON error response returned in the browser. We should improve this to display an HTML-formatted page.

Note: This is a problem with Jira too !69602 (comment 706456391) so a page that could be reused would be great.