wrong driver kind error is not particularly helpful

@nickm writes in introduction.md:

error: expected driver kind struct, but driver is enum (expected kind)

Actually, that error message isn't all that helpful IMO. Maybe it would be better to say something like: "Template Constructor only applies to structs, but we're trying to apply it to an enum."

IOW, I think that it's not so helpful to refer to "drivers" in the error message. -nickm