Allow dsl_deadlist_open() return errors

In some cases like dsl_dataset_hold_obj() it is possible to handle
those errors, so failure to hold dataset should be better than
kernel panic. Some other places where these errors are still not
handled but asserted should be less dangerous just as unreachable.

We have a user report about pool corruption leading to assertions
on these errors. Hopefully this will make behavior a bit nicer.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16836