Show the reason the topic couldn't be split rather than a generic error.

app/assets/javascripts/discourse/controllers/split-topic.js.es6

@@ -54,9 +54,19 @@ export default Discourse.ObjectController.extend(Discourse.SelectedPostsCount, D
         self.send('closeModal');
         self.get('topicController').send('toggleMultiSelect');
         Em.run.next(function() { Discourse.URL.routeTo(result.url); });
-      }, function() {
+      }).catch(function(xhr) {
+
+        var error = I18n.t('topic.split_topic.error');
+
+        if (xhr) {
+          var json = xhr.responseJSON;
+          if (json && json.errors) {
+            error = json.errors[0];
+          }
+        }
+
         // Error moving posts
-        self.flash(I18n.t('topic.split_topic.error'));
+        self.flash(error);
         self.set('saving', false);
       });
       return false;

app/controllers/topics_controller.rb

@@ -275,6 +275,8 @@ class TopicsController < ApplicationController
 
     dest_topic = move_posts_to_destination(topic)
     render_topic_changes(dest_topic)
+  rescue ActiveRecord::RecordInvalid => ex
+    render_json_error(ex)
   end
 
   def change_post_owners

lib/json_error.rb

@@ -5,6 +5,9 @@ module JsonError
     # If we're passed a string, assume that is the error message
     return {errors: [obj]} if obj.is_a?(String)
 
+    # If it's an AR exception target the record
+    obj = obj.record if obj.is_a?(ActiveRecord::RecordInvalid)
+
     # If it looks like an activerecord object, extract its messages
     return {errors: obj.errors.full_messages } if obj.respond_to?(:errors) && obj.errors.present?