bootctl: rework how we handle referenced but absent EFI boot entries
Follow-up for #25368.
Let's consider ENOENT an expected error, and just debug log about it
(though, let's suffix it with `, ignoring.`). All other errors will log
loudly, as they are unexpected errors.
(cherry picked from commit af1bed8e83c3d380d1eb0b9147684b76d1ee4df0)