only try to delete a post from a service if we're able to

lib/postzord/dispatcher.rb

@@ -150,7 +150,7 @@ class Postzord::Dispatcher
       end
     end
     if @object.instance_of?(SignedRetraction)
-      services.each do |service|
+      services.select { |service| service.respond_to? :delete_post }.each do |service|
         Resque.enqueue(Jobs::DeletePostFromService, service.id, @object.target.facebook_id)
       end
     end

spec/lib/postzord/dispatcher_spec.rb

@@ -317,10 +317,18 @@ describe Postzord::Dispatcher do
         retraction = SignedRetraction.build(alice, FactoryGirl.create(:status_message))
         mailman = Postzord::Dispatcher.build(alice, retraction,  :url => "http://joindiaspora.com/p/123", :services => [@service])
 
-        Resque.stub!(:enqueue).with(Jobs::DeletePostFromService, anything, anything)
         Resque.should_receive(:enqueue).with(Jobs::DeletePostFromService, anything, anything)
         mailman.post
       end
+
+      it "doesn't queue a job if we can't delete the post from the service" do
+        retraction = SignedRetraction.build(alice, FactoryGirl.create(:status_message))
+        service = Services::Twitter.new(access_token: "nope")
+        mailman = Postzord::Dispatcher.build(alice, retraction,  :url => "http://joindiaspora.com/p/123", :services => [service])
+
+        Resque.should_not_receive(:enqueue).with(Jobs::DeletePostFromService, anything, anything)
+        mailman.post
+      end
     end
 
     describe '#and_notify_local_users' do