Respond with validation errors in commit status API

If validation errors are present, include validation errors in the
commit status API payload, instead of depending on state machine errors
caused by invalid record.

lib/api/commit_statuses.rb

@@ -78,6 +78,8 @@ module API
           description: params[:description]
         )
+        render_validation_error!(status) if status.invalid?
+
         begin
           case params[:state]
           when 'pending'

spec/requests/api/commit_statuses_spec.rb

@@ -172,7 +172,7 @@ describe API::CommitStatuses, api: true do
         end
       end
-      context 'invalid status' do
+      context 'when status is invalid' do
         before { post api(post_url, developer), state: 'invalid' }
         it 'does not create commit status' do
@@ -180,7 +180,7 @@ describe API::CommitStatuses, api: true do
         end
       end
-      context 'request without state' do
+      context 'when request without a state made' do
         before { post api(post_url, developer) }
         it 'does not create commit status' do
@@ -188,7 +188,7 @@ describe API::CommitStatuses, api: true do
         end
       end
-      context 'invalid commit' do
+      context 'when commit SHA is invalid' do
         let(:sha) { 'invalid_sha' }
         before { post api(post_url, developer), state: 'running' }
@@ -196,6 +196,19 @@ describe API::CommitStatuses, api: true do
           expect(response).to have_http_status(404)
         end
       end
+
+      context 'when target URL is an invalid address' do
+        before do
+          post api(post_url, developer), state: 'pending',
+                                         target_url: 'invalid url'
+        end
+
+        it 'responds with bad request status and validation errors' do
+          expect(response).to have_http_status(400)
+          expect(json_response['message']['target_url'])
+            .to include 'must be a valid URL'
+        end
+      end
     end
     context 'reporter user' do