Add more explicit warning about dedup being dropped

"has unsupported feature: [number]" seems reasonable when we can't
know what the problem was, but with the send -D removal, we know
what it was, and can explicitly tell people "don't do that; try
this if you must".

So let's.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12708