Merge branch '22211-500-instead-of-404' into 'master'

Respond with 404 Not Found for non-existent tags

## What does this MR do?

Handles the case when a non-existent tag is being viewed, and responds with 404 Not Found instead of 500 Internal Server Error.

## Are there points in the code the reviewer needs to double check?

The small changes should be clear from a simple diff.

## Why was this MR needed?

To resolve the issue #22211.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #22211

See merge request !6699

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.13.0 (unreleased)
+  - Respond with 404 Not Found for non-existent tags (Linus Thiel)
   - Truncate long labels with ellipsis in labels page
   - Update runner version only when updating contacted_at
   - Add link from system note to compare with previous version

app/controllers/projects/tags_controller.rb

@@ -20,6 +20,8 @@ class Projects::TagsController < Projects::ApplicationController
   def show
     @tag = @repository.find_tag(params[:id])
 
+    return render_404 unless @tag
+
     @release = @project.releases.find_or_initialize_by(tag: @tag.name)
     @commit = @repository.commit(@tag.target)
   end

spec/controllers/projects/tags_controller_spec.rb

@@ -17,4 +17,18 @@ describe Projects::TagsController do
       expect(assigns(:releases)).not_to include(invalid_release)
     end
   end
+
+  describe 'GET show' do
+    before { get :show, namespace_id: project.namespace.to_param, project_id: project.to_param, id: id }
+
+    context "valid tag" do
+      let(:id) { 'v1.0.0' }
+      it { is_expected.to respond_with(:success) }
+    end
+
+    context "invalid tag" do
+      let(:id) { 'latest' }
+      it { is_expected.to respond_with(:not_found) }
+    end
+  end
 end