Improve system notes for merge-train related auto-merge removals

Release notes

Problem to solve

In #466033 (closed), we ran into unexpected merge train removals, caused by a particular mergeability check failing. The only way to find out was by looking at the rails application logs.

Proposal

• Elaborate on the abort reason in AutoMerge::AddToMergeTrainWhenPipelineSucceedsService#process
  • When the mergeability check fails, it should say which checks failed
  • If it's some other reason, also explain that
• Elaborate on the check if !merge_request.open? || merge_request.broken? || merge_request.draft? in MergeTrains::RefreshMergeRequestService

The engineer prepares the first draft of the messages, but make sure to involve UX and Technical Writing in the merge request review.

Intended users

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