slack.rb

`PostDeployMigrations::Prepare` class raises an exception when on
pending post-migrations are available or when the production checks
fail. The exception is not useful:

* Release managers can't take any action from it, in most of the cases,
  they'll need to wait for a resolution.
* We don't raise exceptions on auto-deploy operations when production
  checks fail. We just notify release managers about it.

This commit updates the way the `Prepare` classes handles exception by:

* Sending a message when there are no pending post-migrations
* Catch the exception when there are no pending post-migrations and when
  production health checks fail
* It still sends a message when production checks fail.

The job that invokes the prepare class will still fail.

As part of gitlab-com/gl-infra/delivery#2441