better donation feature outfit validation

In particular, outfit_id == 0 would cause outfit_id? to
return false, so it wouldn't run the outfit presence
validation, so /donations/features would try to load
outfit #0 and fail.

Also, flash[:alert] instead of flash[:error] when outfit_id
is bad.

app/controllers/donations_controller.rb

@@ -28,7 +28,7 @@ class DonationsController < ApplicationController
         @features.each(&:save!)
       end
     rescue ActiveRecord::RecordInvalid
-      flash[:error] = "Couldn't save donation details. Do those outfits exist?"
+      flash[:alert] = "Couldn't save donation details. Do those outfits exist?"
       redirect_to @donation
     else
       flash[:success] = 'Donation details saved! ' +

app/models/donation_feature.rb

@@ -2,7 +2,7 @@ class DonationFeature < ActiveRecord::Base
   belongs_to :donation
   belongs_to :outfit
 
-  validates :outfit, presence: true, if: :outfit_id?
+  validates :outfit, presence: true, if: :outfit_id_present?
 
   delegate :donor_name, to: :donation
 
@@ -13,4 +13,8 @@ class DonationFeature < ActiveRecord::Base
   def outfit_url=(outfit_url)
     self.outfit_id = outfit_url.split('/').last rescue nil
   end
+
+  def outfit_id_present?
+    outfit_id.present?
+  end
 end