I'm pretty sure there are no inherent jquery-ujs
events regarding form validation, but you can easily use something like jQuery validation to do something only when the form passes client-side validation.
This would probably suit your needs (I don't have much experience with the plugin):
$("#new_import").on('submit', function() {
if($(this).valid()) {
// show your spinner
}
else {
// do other stuff
}
});