Extend test for failed user delete case

When user is the sole owner of the group no explicit delete
options are enabled, the method is expected to return the
attempted delete user with error messages attached.

The test is to ensure we honour this interface and does not
cause unexpected failures from the callers.

spec/services/users/destroy_service_spec.rb

@@ -121,10 +121,17 @@ describe Users::DestroyService do
 
       before do
         solo_owned.group_members = [member]
-        service.execute(user)
+      end
+
+      it 'returns the user with attached errors' do
+        expect(service.execute(user)).to be(user)
+        expect(user.errors.full_messages).to eq([
+          'You must transfer ownership or delete groups before you can remove user'
+        ])
       end
 
       it 'does not delete the user' do
+        service.execute(user)
         expect(User.find(user.id)).to eq user
       end
     end