feat: comment in PR when branch automerge fails (#2058)

This PR adds the feature of commenting on a failed automerge. It's done by adding a conditional in `lib/workers/branch/automerge.js` which, in case of receiving `failure` or `error` from the `getBranchStatus` function, returns the "branch status error" value. Another modification is in `lib/workers/branch/index.js`, which is an adition to the failure response of the `tryBranchAutomerge` function. The added functionality is the ability to add a comment to the PR which had a failure automerging. In case of receiving the aforementioned "branch status error" value, to the comment is appended a note which emphasize the fact that there're multiple failed status checks.

Closes #1934