Try to improve readability of error messages when ImageInput/Output::create fails.
Try to improve readability of error messages when ImageInput/Output::create
fails.


git-svn-id: c3edda122e248ef33995da003806c930f62b9468@1683 31904243-a364-42ac-a3db-9305396518ba

sambler/oiio,jeremyselan/oiio,micler/oiio,mcanthony/oiio,OpenImageIO/oiio,cwilling/oiio,scott-wilson/oiio,bdeluca/oiio,YangYangTL/oiio,bdeluca/oiio,mcanthony/oiio,YangYangTL/oiio,micler/oiio,jeremyselan/oiio,sambler/oiio,cwilling/oiio,scott-wilson/oiio,sambler/oiio,lgritz/oiio,bdeluca/oiio,scott-wilson/oiio,cwilling/oiio,micler/oiio,OpenImageIO/oiio,YangYangTL/oiio,lgritz/oiio,OpenImageIO/oiio,jeremyselan/oiio,mcanthony/oiio,YangYangTL/oiio,mcanthony/oiio,bdeluca/oiio,lgritz/oiio,cwilling/oiio,lgritz/oiio,micler/oiio,OpenImageIO/oiio,scott-wilson/oiio,sambler/oiio