404 on private list link should prompt login

when clicking on a link that leads to a private list's email, Pony mail shows a 404

now, i don't know if it's a breach of data disclosure, but it could also prompt for a login — especially, if a session only expired, rather than never existed.

alternatively, the login form could be integrated into the site's base template, and so, show up everywhere.

though that might require to rethink the design of the login: e.g.: how to choose between different authentication providers.

Reproduce the 404 by opening a private list email link without a valid session, then trace the current error and login handling. Before implementation, resolve whether the desired behavior is a login prompt for expired sessions or a site-wide login form, including how authentication providers are selected; done criteria are not yet defined.

Written by the indexing model from the issue text.