trivial: uefi-dbx: correct error message on missing file
trivial: uefi-dbx: correct error message on missing file

```
unset error in plugin uefi_dbx for add_security_attrs()
```

hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd