FIX: post mover validation color and message (#15688)

When the record is not saved, we should display a proper message.
One potential reason can be plugins for example discourse-calendar is specifying that only first post can contain event

app/assets/javascripts/discourse/app/controllers/move-to-topic.js

@@ -169,7 +169,7 @@ export default Controller.extend(ModalFunctionality, {
           DiscourseURL.routeTo(result.url);
         })
         .catch((xhr) => {
-          this.flash(extractError(xhr, I18n.t("topic.move_to.error")));
+          this.flash(extractError(xhr, I18n.t("topic.move_to.error")), "error");
         })
         .finally(() => {
           this.set("saving", false);

app/controllers/topics_controller.rb

@@ -839,7 +839,7 @@ class TopicsController < ApplicationController
 
     destination_topic = move_posts_to_destination(topic)
     render_topic_changes(destination_topic)
-  rescue ActiveRecord::RecordInvalid => ex
+  rescue ActiveRecord::RecordInvalid, ActiveRecord::RecordNotSaved => ex
     render_json_error(ex)
   end
 

app/models/post_mover.rb

@@ -165,7 +165,7 @@ class PostMover
       guardian: Guardian.new(user),
       skip_jobs: true
     )
-    new_post = @post_creator.create
+    new_post = @post_creator.create!
 
     move_email_logs(post, new_post)