News CI does not fail when there are existing news .rst files

[bobleesj]

Expected behavior:

• The CI should write a comment and fail when there is no news item provided.

Bug:

• When there are existing news items in the main branch, the CI warns but does not fail.

Reference:

• Use diffpy.utils GitHub CI workflows to address codecov 0% problem diffpy/diffpy.snmf#87 (bug found here)
• https://github.com/bobleesj/diffpy.snmf/tree/central-ci/news

[sbillinge]

nice catch