lib-storage: Give a better error message if mail_location has INBOX but no mail root dir.
lib-storage: Give a better error message if mail_location has INBOX but no mail root dir.

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