If a PR author uses the /reviewer credit command to add a reviewer who has already performed an actual review of a PR, the error message is:

"Reviewer <user> has already made an authenticated review of this PR, and does not need to be credited manually."

This makes sense if the review was an approval, but if it was just a comment or a rejection, we should change the wording to something like:

"Reviewer <user> has already made an authenticated review of this PR, but did not approve it. Manually crediting them is not allowed."