Log invalid branch name if creation fails

This would have made it easier to diagnose errors in
https://gitlab.com/gitlab-org/gitlab/issues/34310.

app/services/create_branch_service.rb

@@ -14,7 +14,7 @@ class CreateBranchService < BaseService
     if new_branch
       success(new_branch)
     else
-      error('Invalid reference name')
+      error("Invalid reference name: #{branch_name}")
     end
   rescue Gitlab::Git::PreReceiveError => ex
     error(ex.message)

spec/requests/api/branches_spec.rb

@@ -602,7 +602,7 @@ describe API::Branches do
       post api(route, user), params: { branch: 'new_design3', ref: 'foo' }
 
       expect(response).to have_gitlab_http_status(400)
-      expect(json_response['message']).to eq('Invalid reference name')
+      expect(json_response['message']).to eq('Invalid reference name: new_design3')
     end
   end
 

spec/services/create_branch_service_spec.rb

@@ -22,5 +22,20 @@ describe CreateBranchService do
         expect(project.repository.branch_exists?('my-feature')).to be_truthy
       end
     end
+
+    context 'when creating a branch fails' do
+      let(:project) { create(:project_empty_repo) }
+
+      before do
+        allow(project.repository).to receive(:add_branch).and_return(false)
+      end
+
+      it 'retruns an error with the branch name' do
+        result = service.execute('my-feature', 'master')
+
+        expect(result[:status]).to eq(:error)
+        expect(result[:message]).to eq("Invalid reference name: my-feature")
+      end
+    end
   end
 end