tools: mkimage: don't print error message "Success" in case of failure

In case there's no struct image_type_params::set_header callback, no
"errno" will be set. Don't fail with an error message, followed by
"Success". Remove the printing of the human readable "errno" value.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Reviewed-by: Simon Glass <sjg@chromium.org>