Send back form on validation error of multi image uploader

This allows validations errors to be displayed on the frontend

wagtail/wagtailimages/templates/wagtailimages/multiple/confirmation.json

@@ -1,4 +1,7 @@
 {
     "image_id": {{ image.id }},
+{% if form %}
+    "form": "{% filter escapejs %}{% include 'wagtailimages/multiple/edit_form.html' %}{% endfilter %}",
+{% endif %}
     "success": {% if success %}true{% else %}false{% endif %}
 }

wagtail/wagtailimages/views/multiple.py

@@ -57,6 +57,7 @@ def edit(request, image_id, callback=None):
     return render(request, 'wagtailimages/multiple/confirmation.json', {
         'success': False,
         'image': image,
+        'form': form,
     }, content_type='application/json')