efi_loader: improve efi_var_from_file() description

It is unclear to developers why efi_var_from_file() returns EFI_SUCCESS if
file ubootefi.var is missing or corrupted. Improve the description.

Reported-by: Weizhao Ouyang <o451686892@gmail.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Weizhao Ouyang <o451686892@gmail.com>