Comment on PRs if a remote call to merge a change failed

This adds a comment to PRs when a successfully tested change fails
to merge into an upstream repository due to a Github API error. It
uses the merge_failure_message to format said message.

Change-Id: Ieed299a71133ebd88e2c31d6bd0c54390c0901ba
Signed-off-by: Adam Gandelman <adamg@ubuntu.com>
diff --git a/tests/fixtures/layouts/merging-github.yaml b/tests/fixtures/layouts/merging-github.yaml
index 4e13063..9f43f75 100644
--- a/tests/fixtures/layouts/merging-github.yaml
+++ b/tests/fixtures/layouts/merging-github.yaml
@@ -2,6 +2,7 @@
     name: merge
     description: Pipeline for merging the pull request
     manager: independent
+    merge-failure-message: 'Merge failed'
     trigger:
       github:
         - event: pull_request