zpool: bogus error for invalid dedupditto value

When provided with an invalid 'dedupditto' value zpool prints
a misleading error message:

    $ sudo zpool set dedupditto=99 pp
    cannot set property for 'pp': property 'dedupditto'(14) not defined

Fix this by printing a meaningful error description for unsupported
'dedupditto' values.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8079