Introduce failure_reason or failure_code for pipeline

The following discussion from gitlab-ce!11910 should be addressed:

• @ayufan started a discussion: (+6 comments)

  I wonder if we should not do it differently:

  class Pipeline
    enum :failure_reason [
      everything_ok: null,
      unauthorized: 10
    ]
  end
  
  class CreatePipelineService
    def execute
      unless check_permission?
        return error!(:unauthorized, ...)
      end
    end
  end
  
  class PipelineScheduleWorker
    def perform
       for_each_Schedule do |schedule|
         pipeline = CreatePipelineService.new.execute
         schedule.deactivate! if pipeline.unauthorized?
       end
    end
  end