Add a test case for 404 and refactor docs as per feedback

doc/api/error_tracking.md

-# Error Tracking Settings API
+# Error Tracking settings API
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/34940) in GitLab 12.7.
 
-## Error Tracking Project Settings
+## Error Tracking project settings
 
-The Project Settings API allows you to retrieve Error Tracking Settings for a Project. Only for project maintainers.
+The project settings API allows you to retrieve the Error Tracking settings for a project. Only for project maintainers.
 
-### Retrieve Error Tracking Settings
+### Get Error Tracking settings
 
 ```
 GET /projects/:id/error_tracking/settings
@@ -14,7 +14,7 @@ GET /projects/:id/error_tracking/settings
 
 | Attribute | Type    | Required | Description           |
 | --------- | ------- | -------- | --------------------- |
-| `id`      | integer | yes      | The ID of the project owned by the authenticated user |
+| `id`      | integer | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user |
 
 ```bash
 curl --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/projects/1/error_tracking/settings

lib/api/error_tracking.rb

@@ -10,6 +10,7 @@ module API
 
     resource :projects, requirements: API::NAMESPACE_OR_PROJECT_REQUIREMENTS do
       desc 'Get error tracking settings for the project' do
+        detail 'This feature was introduced in GitLab 12.7.'
         success Entities::ErrorTracking::ProjectSetting
       end
 

spec/requests/api/error_tracking_spec.rb

@@ -29,6 +29,22 @@ describe API::ErrorTracking do
       end
     end
 
+    context 'without a project setting' do
+      let(:project) { create(:project) }
+
+      before do
+        project.add_maintainer(user)
+      end
+
+      it 'returns 404' do
+        make_request
+
+        expect(response).to have_gitlab_http_status(:not_found)
+        expect(json_response['message'])
+          .to eq('404 Error Tracking Setting Not Found')
+      end
+    end
+
     context 'when authenticated as reporter' do
       before do
         project.add_reporter(user)