<div id="formError" class="modal fade" style="font-family: Roboto;">
<div class="modal-dialog">
......
......
$('button').on('click',function(){
$('#formError').addClass('in');
});
<button data-toggle="modal" data-target="#formError">открыть</button>
$('button').on('click',function(){
$('#formError').modal('show');
});