9072 handle error of zap_cursor_retrieve() for log spacemap zap

In spa_ld_log_sm_metadata(), it is possible for zap_cursor_retrieve()
to return errors other than the expected ENOENT (e.g. when we are at
the end of the zap). Ensure that these error cases are handled
correctly by the import path.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Sara Hartse <sara.hartse@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #9074