Send Discord notifications for rollback failures

[NDB-7]

For issue #34

Steps to test

1. Ensure enable_rollback is true in config.yml
2. On the repo being watched, make a new branch locally but don't push it to the remote repo (this will trigger a Git error)
3. On that repo, modify docker-compose.yml in a way that makes docker-compose up --build -d trigger an error
4. Run python server.py
5. Trigger a webhook event with curl on that repo and local branch:

curl -X POST WEBHOOK_URL \
  -H "Content-Type: application/json" \
  -H "X-GitHub-Event: push" \
  -d '{
    "ref": "refs/heads/BRANCH_NAME",
    "repository": {
      "name": "REPO_NAME"
    }
  }'

After the webhook event, the Git step will trigger an error, initiating a rollback. During the rollback, the Docker step will fail, resulting in a rollback failure.

Here's my config:

Logs

Command line output:

Discord embed output:

[adarshm11]

lets keep the try/catch/finally block here because without it there will be uncaught exceptions from the subprocess runs