[web] return 403 for Unauthorized, not 401
401 is reserved to HTTP authentication. Just because it's also called
"unauthorized" doesn't mean it's the same as cubicweb's Unauthorized
exception. Closes #3648809.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html