portable: add a few more useful debug log messages
    
    When attaching and /etc/systemd/system.attached can't be created or used
    (eg: dead symlink) the logs are pretty much useless as even at debug
    level there's no indication of what is going wrong.
    Add some debug logs, and return a more specific error string over D-Bus.
    
    (cherry picked from commit 80d95fcd6e1947a7887b96b22a32dbca115baac9)