Change ImageInput and ImageOutput's error() method from protected to public.
Change ImageInput and ImageOutput's error() method from protected to public.

This lets a calling program set an error for the II or IO.

bdeluca/oiio,mcanthony/oiio,bdeluca/oiio,cwilling/oiio,mcanthony/oiio,bdeluca/oiio,lgritz/oiio,micler/oiio,YangYangTL/oiio,mcanthony/oiio,lgritz/oiio,cwilling/oiio,lgritz/oiio,YangYangTL/oiio,OpenImageIO/oiio,OpenImageIO/oiio,mcanthony/oiio,micler/oiio,YangYangTL/oiio,micler/oiio,lgritz/oiio,bdeluca/oiio,cwilling/oiio,micler/oiio,OpenImageIO/oiio,OpenImageIO/oiio,YangYangTL/oiio,cwilling/oiio