FIX: `PG::UniqueViolation` when trying to use the same embed code

Previously providing an embed code already in use would result in
a logged server error. After this commit the error is gracefully
bubbled up from the `PostCreator`

app/models/topic_embed.rb

@@ -4,6 +4,7 @@ class TopicEmbed < ActiveRecord::Base
   belongs_to :topic
   belongs_to :post
   validates_presence_of :embed_url
+  validates_uniqueness_of :embed_url
 
   def self.normalize_url(url)
     url.downcase.sub(/\/$/, '').sub(/\-+/, '-').strip

lib/post_creator.rb

@@ -193,7 +193,8 @@ class PostCreator
   # discourse post.
   def create_embedded_topic
     return unless @opts[:embed_url].present?
-    TopicEmbed.create!(topic_id: @post.topic_id, post_id: @post.id, embed_url: @opts[:embed_url])
+    embed = TopicEmbed.new(topic_id: @post.topic_id, post_id: @post.id, embed_url: @opts[:embed_url])
+    rollback_from_errors!(embed) unless embed.save
   end
 
   def handle_spam

spec/components/post_creator_spec.rb

@@ -559,7 +559,17 @@ describe PostCreator do
                                 title: 'Reviews of Science Ovens',
                                 raw: 'Did you know that you can use microwaves to cook your dinner? Science!')
       creator.create
+      expect(creator.errors).to be_blank
       expect(TopicEmbed.where(embed_url: embed_url).exists?).to eq(true)
+
+      # If we try to create another topic with the embed url, should fail
+      creator = PostCreator.new(user,
+                                embed_url: embed_url,
+                                title: 'More Reviews of Science Ovens',
+                                raw: 'As if anyone ever wanted to learn more about them!')
+      result = creator.create
+      expect(result).to be_present
+      expect(creator.errors).to be_present
     end
   end