Cherry-pick bot should just add author to attention set on failure

Currently the cherry-pick bot is a bit too chatty when cherry-pick integrations fail.

It keeps posting 'Attention set updated: Integration for this cherry-pick failed to merge. Added patch author'
messages every time the patch integration fails, but the author was already added as the author the first time the integration failed.

It would be nice if the bot stopped posting that comment on subsequent failures.

It should suffice to add the author to the attention set upon subsequent failures, and only post the comment the first time the patch fails and the author wasn't part of the reviewers.

This reduces the number of emails that are being sent every time a whole chain of cherry-picks fails to integrate.