systemctl: avoid spurious warning about missing reboot-param file

$ reboot -f
Failed to read reboot parameter file: No such file or directory

It seems that the warning on ENOENT was inadvertently introduced in
27c06cb5.

The warning reported in #5646 comes from systemctl, but let's fix the other
call site in the same way too.

Fixes #5646.

(cherry picked from commit 19fbf49c)