Improve error message of zfs redact

We improve the error message of zfs redact by checking if the target
snapshot exists, and if all the redaction snapshots exist. As a
future improvement we could iterate over every snapshot provided and
use that to determine which one specifically doesn't exist.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #11426
Closes #14496