YCgCo decoding works just fine. It depends on the API user what is done
with the output. Some API users might support it, some not. Some users
might support it under certain circumstances only.

It is not the job of the decoder to print this message. If the API user
supports it, this message is extremely unhelpful.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>