Refactor `pipeline.errors.full_messages.join(',')` to be `pipeline.errors_message`

It's just method refactoring.

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The following discussion from !13278 (merged) should be addressed:

• @ayufan started a discussion: (+2 comments)

  Can we have this pipeline.errors.full_messages.join(',') as a method of pipeline? Like pipeline.errors_message? I see that we use this construct already elsewhere.