Modal forms now display non-field errors

- Critical for errors not relating to a particular field (e.g. unique_together)
- Ref: https://stackoverflow.com/questions/7419535/how-to-render-django-form-errors-not-in-a-ul

InvenTree/templates/modal_form.html

@@ -1,5 +1,14 @@
+{% if form.non_field_errors %}
+<div class='alert alert-danger' role='alert' style='display: block;'>
+  <b>Error Submitting Form:</b>
+  {{ form.non_field_errors }}
+</div>
+{% endif %}
+
 <form method="post" action='' class='js-modal-form' enctype="multipart/form-data">
   {% csrf_token %}
   {% load crispy_forms_tags %}
+  
+  
   {% crispy form %}
 </form>