[ew] Block library on protected Sites shows a generic 401 error when not authenticated via sidekick

Ideally (not sure if this works) we use a DA token exchange flow to request the library content from aem.page so if you are logged into DA, you don't ever see this 401.
If that isn't possible or won't work, the error we display should tell the user what to do (open the page and login with sidekick)

Reproduce the block-library request on a protected Site while unauthenticated through Sidekick, then trace the request and 401 error handling in the relevant entry points. Determine whether the DA token exchange with aem.page is feasible; otherwise, done means the error clearly directs the user to open the page and log in with Sidekick.

Written by the indexing model from the issue text.