Reject zfs send -RI with nonexistent fromsnap

Right now, zfs send -I dataset@nonexistent dataset@existent fails, but
zfs send -RI dataset@nonexistent dataset@existent does not.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12574
Closes #12575