shutdown: rework messages during shutdown

When running in 'quiet' mode, the only message printed from shutdown
binary would be 'Cannot finalize remaining file systems and devices,
giving up.', the only log line at error level before switch back to
initramfs. This is misleading, because in initramfs everything will
be cleaned up properly.

Avoid printing anything at error level before the attempt to switch
back to initramfs. Rework the messages to contain a bit more
information what is still remaining, to help people diagnose shutdown
issues.