Fix error propagation from lzc_send_redacted

Any error from lzc_send_redacted is overwritten by the error of
send_conclusion_record; skip writing the conclusion record if there
was an earlier error.

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Philipp Riederer <philipp@riederer.email>
Closes #12766