Have you tried marking the question as required? All select one items have an option entitled "Answer is required", in the case of select many questions you will need to specify a minimum number of options to select, and when it is enabled a couple of things occur.
1 – The item is marked with an * to denote that it is required.
2 - Respondents are unable to proceed to the next page unless the question is answered.
3 – In the event that a respondent attempts to proceed without answering all required question an error message is displayed at the top of the page as well as error messages next to each unanswered question.
If you would like to make the error text more noticeable I would suggest applying a style to your survey. If you have questions regarding styles please review our training video on the subject
http://www.prezzatech.com/training/ .