Add messages when trying to add a rider that is not available.

Bug 486220. It would be helpful to add messaging when trying to add a rider or benefit to a case that isn't available to advise the user why they can't add the rider/benefit. On case LB09253799 I didn't realize that Critical Illness Rider wasn't available on term case for California and spent time researching why we couldn't add it. Additional messaging would make it more clear to the user why they are not able to add the rider/benefit and reduce questions.