Catch the post-deploy prepare exception

One of the initial jobs of the post-deploy pipeline is the prepare job, which executes the AutoDeploy::PostMigrations::Prepare class. This class is in charge of allowing or preventing the execution of the post-deploy pipeline. The class fails when:

• There are no pending post-migrations, or
• The production checks fail

In both scenarios the exception raised is not taken care by the application, which doesn't make much sense:

• There's no action for release managers to take based on that exception. They just need to wait for a post-migration or for the incident/deployment to finish
• The exception sends a message to the #f_upcoming_release channel. For auto-deploy, we sent a message to slack notifying release managers about it, but we don't raise an error.

Let's handle the exception in the same way auto-deploy handles it: Send a message to slack, informing release managers about the problem.

added AutoDeploy DeliveryP2 post-deploy migrationsphase 1 teamDelivery labels

added to epic &585 (closed)

assigned to @mayra-cabrera

mentioned in issue #2352 (closed)

added workflow-infraTriage label

added workflow-infraReady label and removed workflow-infraTriage label

mentioned in commit gitlab-org/release-tools@aa0ae9bc

mentioned in merge request gitlab-org/release-tools!1900 (merged)

mentioned in commit gitlab-org/release-tools@4a309fa4

mentioned in commit gitlab-org/release-tools@da1f1ef4

added Delivery teamBuild label

mentioned in commit gitlab-org/release-tools@2575d275

mentioned in commit gitlab-org/release-tools@7070cc25

mentioned in commit gitlab-org/release-tools@79af136b

mentioned in commit gitlab-org/release-tools@2dd0f03d

mentioned in merge request gitlab-org/release-tools!1903 (merged)

mentioned in merge request gitlab-org/release-tools!1905 (merged)

With gitlab-org/release-tools!1905 (merged) this issue is completed.

closed

changed milestone to %15.2