Get rid of confusing/bogus ImageInput error (#2126)
Get rid of confusing/bogus ImageInput error (#2126)

read_tiles was calling all errors "no support for format", but that's
really weird. If it's a bad tile read, the error will already have
been set by the underlying read_native_tiles.
OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio