lib-storage: Handle better when attempting to access shared mailboxes for nonexistent users.
lib-storage: Handle better when attempting to access shared mailboxes for nonexistent users.

jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch