Linking to emails that require authentication is confusing

When linking to an email that requires authentication, the UI shows the "An error occured" due to the 404 error reported by https://lists.apache.org/api/thread.lua . Users must first close the error box, and then use the 'profile' icon in the top-right to log in. This turns out to be non-obvious to some users, causing them to assume links to important information are broken and ignoring them.

I think it would be good to show page that makes it more obvious that you might need to log in on 404, or perhaps even consider returning a 401 from thread.lua in this case and redirect people to the login page automatically.

Start with the email-link handling around https://lists.apache.org/api/thread.lua and the login flow reached through the profile icon. Determine how authenticated email links produce the 404 and choose a clear user-facing outcome, such as login guidance or redirecting to authentication; done means users no longer mistake these links for broken content.

Written by the indexing model from the issue text.