shutdown: Reduce log level of unmounts

shutdown: Reduce log level of unmounts

There is little point in logging about unmounting errors if the
exact mountpoint will be successfully unmounted in a later retry
due unmounts below it having been removed.

Additionally, don't log those errors if we are going to switch back
to a initrd, because that one is also likely to finalize the remaining
mountpoints. If not, it will log errors then.