Caught while reviewing 18323.

Although models throw error, they are not handled.
Not even logged.

service/CopyQuestion.php

    public function copyQuestion($copyOptions)
    {
        $copySuccessful = $this->createNewCopiedQuestion(
            $this->copyQuestionValues->getQuestionCode(),
            $this->copyQuestionValues->getQuestionGroupId(),
            $this->copyQuestionValues->getQuestiontoCopy()
        );
        if ($copySuccessful) {
            //copy question languages
            $this->copyQuestionLanguages($this->copyQuestionValues->getQuestiontoCopy(), $this->copyQuestionValues->getQuestionL10nData());

            //copy subquestions
            if ($copyOptions['copySubquestions']) {
                $this->copyQuestionsSubQuestions($this->copyQuestionValues->getQuestiontoCopy()->qid);
            }
      ...
      ...

No checking of errors after save()

    private function copyQuestionsSubQuestions($parentId)
    {
        //copy subquestions
        $areSubquestionsCopied = true;
        $subquestions = \Question::model()->findAllByAttributes(['parent_qid' => $parentId]);

        foreach ($subquestions as $subquestion) {
            $copiedSubquestion = new \Question();
            $copiedSubquestion->attributes = $subquestion->attributes;
            $copiedSubquestion->parent_qid = $this->newQuestion->qid;
            $copiedSubquestion->qid = null; //new question id needed ...
            $areSubquestionsCopied = $areSubquestionsCopied && $copiedSubquestion->save();
            foreach ($subquestion->questionl10ns as $subquestLanguage) {
                $newSubquestLanguage = new \QuestionL10n();
                $newSubquestLanguage->attributes = $subquestLanguage->attributes;
                $newSubquestLanguage->qid = $copiedSubquestion->qid;
                $newSubquestLanguage->id = null;
                $newSubquestLanguage->save();
            }
        }

        return $areSubquestionsCopied;
    }