AZ714: Sign in first

A call that needs someone signed in ran in a transaction signed in as nobody (authz.act_as(NULL, NULL), a visitor who hasn't signed in): an access request, or an API key made for oneself. SQLSTATE 42501; answer 401, as the SDKs do, with the database's words. A transaction that never signed in says AZ701.

Reported to the app, by the authz.* functions and the policy's rules: the code is in the error's HINT (rowstile help AZ714), and the SDKs' errors carry it as code.