Use pipelines.errors when communicating the error

app/controllers/projects/pipelines_controller.rb

@@ -15,19 +15,17 @@ class Projects::PipelinesController < Projects::ApplicationController
   end
 
   def new
+    @pipeline = project.ci_commits.new
   end
 
   def create
-    begin
-      pipeline = Ci::CreatePipelineService.new(project, current_user, create_params).execute
-      redirect_to namespace_project_pipeline_path(project.namespace, project, pipeline)
-    rescue ArgumentError => e
-      flash[:alert] = e.message
-      render 'new'
-    rescue
-      flash[:alert] = 'The pipeline could not be created. Please try again.'
+    @pipeline = Ci::CreatePipelineService.new(project, current_user, create_params).execute
+    unless @pipeline.persisted?
       render 'new'
+      return
     end
+
+    redirect_to namespace_project_pipeline_path(project.namespace, project, @pipeline)
   end
 
   def show

app/services/ci/create_pipeline_service.rb

 module Ci
   class CreatePipelineService < BaseService
     def execute
+      pipeline = project.ci_commits.new
+
       unless ref_names.include?(params[:ref])
-        raise ArgumentError, 'Reference not found'
+        pipeline.errors.add(:base, 'Reference not found')
+        return pipeline
       end
 
       unless commit
-        raise ArgumentError, 'Commit not found'
+        pipeline.errors.add(:base, 'Commit not found')
+        return pipeline
       end
 
       unless can?(current_user, :create_pipeline, project)
-        raise RuntimeError, 'Insufficient permissions to create a new pipeline'
+        pipeline.errors.add(:base, 'Insufficient permissions to create a new pipeline')
+        return pipeline
       end
 
-      pipeline = new_pipeline
-
+      begin
         Ci::Commit.transaction do
+          pipeline.sha = commit.id
+          pipeline.ref = params[:ref]
+          pipeline.before_sha = Gitlab::Git::BLANK_SHA
+
           unless pipeline.config_processor
-          raise ArgumentError, pipeline.yaml_errors || 'Missing .gitlab-ci.yml file'
+            pipeline.errors.add(:base, pipeline.yaml_errors || 'Missing .gitlab-ci.yml file')
+            raise ActiveRecord::Rollback
           end
 
           pipeline.save!
           pipeline.create_builds(current_user)
         end
+      rescue
+        pipeline.errors.add(:base, 'The pipeline could not be created. Please try again.')
+      end
 
       pipeline
     end
 
     private
 
-    def new_pipeline
-      project.ci_commits.new(sha: commit.id, ref: params[:ref], before_sha: Gitlab::Git::BLANK_SHA)
-    end
-
     def ref_names
       @ref_names ||= project.repository.ref_names
     end

app/views/projects/pipelines/new.html.haml

 - page_title "New Pipeline"
 = render "header_title"
 
-- if @error
-  .alert.alert-danger
-    %button{ type: "button", class: "close", "data-dismiss" => "alert"} ×
-    = @error
 %h3.page-title
   New Pipeline
 %hr
 
-= form_tag namespace_project_pipelines_path, method: :post, id: "new-pipeline-form", class: "form-horizontal js-new-pipeline-form js-requires-input" do
+= form_for @pipeline, url: namespace_project_pipelines_path(@project.namespace, @project), html: { id: "new-pipeline-form", class: "form-horizontal js-new-pipeline-form js-requires-input" } do
+  = form_errors(@pipeline)
   .form-group
     = label_tag :ref, 'Create for', class: 'control-label'
     .col-sm-10

spec/features/pipelines_spec.rb

@@ -126,14 +126,21 @@ describe "Pipelines" do
     before { visit new_namespace_project_pipeline_path(project.namespace, project) }
 
     context 'for valid commit' do
-      before do
-        fill_in('Create for', with: 'master')
-        stub_ci_commit_to_return_yaml_file
-      end
+      before { fill_in('Create for', with: 'master') }
+
+      context 'with gitlab-ci.yml' do
+        before { stub_ci_commit_to_return_yaml_file }
 
         it { expect{ click_on 'Create pipeline' }.to change{ Ci::Commit.count }.by(1) }
       end
 
+      context 'without gitlab-ci.yml' do
+        before { click_on 'Create pipeline' }
+
+        it { expect(page).to have_content('Missing .gitlab-ci.yml file') }
+      end
+    end
+
     context 'for invalid commit' do
       before do
         fill_in('Create for', with: 'invalid reference')