Incorrect error message on y8 input

[tp7]

c=BlankClip(pixel_type="YV12")
c.ConvertToY8().mt_merge(c,c.ConvertToY8(), u=3, v=3, luma=true)

Outputs "luma is not supported in 422".

[tophf]

'luma' parameter is not supported for input clip without chroma channel

is this better or

'luma' parameter is ignored for input clip without chroma channel

?

[tp7]

First one works. I'll add it together with some more major bugfix if discovered.