[9.20] chg: doc: Make empty changelog fatal error

The prep_doc_mr.py script of the bind9-qa repo needs a way to know that
gitchangelog.py did not produce entries. In the case of release notes,
it dies with "No commits matching given revlist". For changelog entries
it used to warn about "Empty changelog", but did not return non-zero
exit code.

Backport of MR !10591

Merge branch 'backport-mnowak/make-empty-changelog-fatal-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10641