[TASK] validateRstFiles script: improve error message

Improve the error message for non valid index keywords in
changelog RST files.

Resolves: #80310
Releases: master
Change-Id: Ibca4f135a7f43c7c936fd57c14b1f7b7fc0a9d1f
Reviewed-on: https://review.typo3.org/52068


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>