Expose build failure reason

We can use this to show more informative error messages with
links to documentation etc.

app/serializers/build_details_entity.rb

@@ -45,6 +45,8 @@ class BuildDetailsEntity < JobEntity
     erase_project_job_path(project, build)
   end
+  expose :failure_reason, if: -> (*) { build.failed? }
+
   expose :terminal_path, if: -> (*) { can_create_build_terminal? } do |build|
     terminal_project_job_path(project, build)
   end

spec/serializers/build_details_entity_spec.rb

@@ -112,5 +112,15 @@ describe BuildDetailsEntity do
         expect(subject['merge_request_path']).to be_nil
       end
     end
+
+    context 'when the build has failed' do
+      let(:build) { create(:ci_build, :created) }
+
+      before do
+        build.drop!(:unmet_prerequisites)
+      end
+
+      it { is_expected.to include(failure_reason: 'unmet_prerequisites') }
+    end
   end
 end