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>
3 files changed