Since commit 934f2d2f,
cmdutils_read_file() prints a confusing message on success:

IO error: Success

This is because the error message is printed on the success path as
well. Add the missing condition so that it is only printed on error.
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>