Show error message when pipelines disabled and Pipeline must succeed

This is the first step towards fixing the Epic #12686 (closed) to show accurate messages when MR button is disabled. This specific issue should not focus on the problem as a whole, but a very specific instance where the settings are simply invalid and we give no help to users.

• Pipelines are disabled
• MR pipeline must succeed is turned on
• No external CI is setup

This correspond to option C in the EPIC:

C. Pipelines are disabled and there is no pipeline information available for the commit (pipelines were not enabled or there was no ci configuration when the commit was created):

• Settings -> General -> Merge Requests -> Merge checks -> "Pipelines must succeed" is enabled
• Settings -> General -> Visibility, project features, permissions -> "Pipelines" is disabled
• Pipeline information is generally not shown anymore except in merge requests where it is required to succeed.
• Pipeline section of merge request widget states: No pipeline was run for this commit.
• The merge section of the merge request widget states: Pipelines must succeed for merge requests to be eligible to merge. Please enable pipelines for this project to continue. For more information, see the documentation.

Screenshots

Before

After

changelogs/unreleased/216031-show-accurate-error-message-when-pipelines-disabled-mr-must-succee.yml

+---
+title: Add clear explanation to the MR widget when no CI is available and Pipeline
+  must succeed option is activated
+merge_request:
+author:
+type: changed