efibootmgr: Use returned error code for error message, not errno

[freqlabs]

Summary

efivar_unix_path_to_device_path returns the error code, it does not set errno.

Test Plan

Before:

➜  ~ sudo efibootmgr -a -c -l /mnt/EFI/freebsd/loader.efi -L nvme0-FreeBSD
efibootmgr: Cannot translate unix loader path '\mnt\EFI\freebsd\loader.efi' to UEFI: No error: 0

After:

➜  ~ sudo efibootmgr -a -c -l /mnt/EFI/freebsd/loader.efi -L nvme0-FreeBSD
efibootmgr: Cannot translate unix loader path '\mnt\EFI\freebsd\loader.efi' to UEFI: No such file or directory