form_set_error highlight offending field?
Don't put the [value] inside, it should work with only 

form_set_error('field_reviewer_terms_of_service', t('You must agree to the Terms of Service.'));


See the documentation for form_set_error().
You may have to use some [value inside your field if it is multivalued, see this comment but we need to see the HTML ouput for your field.