Improve specs for pipeline failure reason presenter

Conflicts:
	app/presenters/ci/pipeline_presenter.rb

app/presenters/ci/pipeline_presenter.rb

 module Ci
   class PipelinePresenter < Gitlab::View::Presenter::Delegated
-    FAILURE_REASONS = {}
+    FAILURE_REASONS = {
+      config_error: 'CI/CD YAML configuration error!'
+    }.freeze
+
+    presents :pipeline
 
     def failure_reason
       return unless pipeline.failure_reason?

spec/presenters/ci/pipeline_presenter_spec.rb

@@ -51,4 +51,21 @@ describe Ci::PipelinePresenter do
       end
     end
   end
+
+  context '#failure_reason' do
+    context 'when pipeline has failure reason' do
+      it 'represents a failure reason sentence' do
+        pipeline.failure_reason = :config_error
+
+        expect(presenter.failure_reason)
+          .to eq 'CI/CD YAML configuration error!'
+      end
+    end
+
+    context 'when pipeline does not have failure reason' do
+      it 'returns nil' do
+        expect(presenter.failure_reason).to be_nil
+      end
+    end
+  end
 end

spec/serializers/pipeline_entity_spec.rb

@@ -108,5 +108,18 @@ describe PipelineEntity do
         expect(subject[:ref][:path]).to be_nil
       end
     end
+
+    context 'when pipeline has a failure reason set' do
+      let(:pipeline) { create(:ci_empty_pipeline) }
+
+      before do
+        pipeline.drop!(:config_error)
+      end
+
+      it 'has a correct failure reason' do
+        expect(subject[:failure_reason])
+          .to eq 'CI/CD YAML configuration error!'
+      end
+    end
   end
 end