Add option to raise error for PRs with missing labels (instead of just warning)

[joshuacwnewton]

This would allow a GH Action created using changelog (example) to halt midway, then provide a list of PRs that still need labels.

Context:

changelog/changelog/changelog.py

Lines 238 to 240 in 728e330

	diff_pr = all_pr - changelog_pr
	for diff in diff_pr:
	logger.warning('Pull request not labeled: %s', diff)