Emit explanatory error message for iteration on CartesianIndex

[timholy]

As suggested in #23982. Sometimes these messages are helpful, but they can occasionally cause problems. In this case the most confusing part of the message would be if someone hit this in the context of

julia> I = CartesianIndex(1,2,3)
CartesianIndex(1, 2, 3)

julia> for i in I
           @show i
       end
ERROR: iteration is deliberately unsupported for CartesianIndex. Use `I` rather than `I...`, or use `Tuple(I)...`

and the user might wonder what splatting has to do with it. Nevertheless I suspect that splatting is how most people hit this issue.

But I am not attached to this. I've also deliberately not added a test for this behavior, since once the length can be inferred we will want to support iteration over the elements of CartesianIndex.

[timholy]

Barring objections I'll merge soon.