Notify user when merge fails

[asvoboda]

Currently, when bulldozer fails to merge it does so silently behind the scenes.

Instead, we can post a comment to the PR to indicate what happened. The error string included from GH is quite helpful in this case, and we log it already.

Merge rejected due to unsatisfied condition: \"You're not authorized to push to this branch. Visit https://help.github.com/enterprise/2.20/user/articles/about-protected-branches/ for more information.\