[changes] RFC: Only post comment for failing builds

Summary:
I was looking at https://tails.corp.dropbox.com/p/changesbot/

I am not sure if just this may be too spammy. Since this is a new addition, I feel starting with just failed builds may be less disruptive. Thoughts?

Test Plan: unit tests

Reviewers: cramer

Subscribers: alexallain

Differential Revision: https://tails.corp.dropbox.com/D71409