GitLab for Slack notification: Include pipeline creation failure

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Proposal

Currently, the GitLab for Slack app will notify if a pipeline fails. However as it replies on the status changing, it does not notify if the pipeline fails to be created.

Example: https://gitlab.com/gitlab-com/content-sites/handbook/-/pipelines/1480650935

Proposing we add pipeline failures where pipeline has not yet started to run.