tmpfiles: log at info level when some allowed failures occur

tmpfiles: log at info level when some allowed failures occur

In provision.conf we ship:

d- /root :0700 root :root -
d- /root/.ssh :0700 root :root -

These are allowed to fail, for example on a read-only filesystem. But they still
log at error level, which is annoying and gets flagged. Tune those specific errors
down to info.

There are likely more that could be tuned down, but the important thing is to cover
the tmpfiles.d that we ship right now.

Before:

$ echo -e "d- /root :0700 root :root - \nd- /root/.ssh :0700 root :root -" | SYSTEMD_LOG_LEVEL=err build/systemd-tmpfiles --root=/tmp/img --create -
Failed to create directory or subvolume "/tmp/img/root": Read-only file system
Failed to open path '/tmp/img/root': No such file or directory
$

After:

$ echo -e "d- /root :0700 root :root - \nd- /root/.ssh :0700 root :root -" | SYSTEMD_LOG_LEVEL=err build/systemd-tmpfiles --root=/tmp/img --create -
$