[9.20] fix: usr: Raise the log level of priming failures

When a priming query is complete, it's currently logged at level ISC_LOG_DEBUG(1), regardless of success or failure. We are now raising it to ISC_LOG_NOTICE in the case of failure. [GL #3516]

Closes #3516

Backport of MR !9121

Merge branch 'backport-3516-log-priming-errors-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9250