Raise an error when automation Envoy->Proxy->Istio fails

When a change is made in Envoy, it needs to be propagated to Proxy, and when it is merged in Proxy, Istio also needs to be changed. This process is currently automated and described here: https://github.com/maistra/proxy/tree/maistra-2.2/maistra#relationship-with-envoy-and-istio-automation

We need to enhance it so that if it fails for any reason, we are notified that a failure happened. This is to avoid the situation where a PR is merged in Envoy and it never reaches Proxy, risking the product to be built and released without containing the actual Envoy changes.

This notification must be noisy so that it's not unnoticed by us. Upon receiving it, we should stop what we are doing and take action. Most of the times the action will be to perform the steps manually,fixing eventual problems and creating a manual PR updating Proxy or Istio repos.

Acceptance criteria:

• Create a new job that compares the SHA of the proxy and Envoy.
• If SHA doesn't match, istio-team must be notified via email or slack.