[rackspace] fixing broken tests

lib/fog/rackspace/requests/compute/delete_image.rb

@@ -34,7 +34,7 @@ module Fog
             response
           else
             response.status = 400
-            raise(Excon::Errors.status_error({:expects => 202}, response))
+            raise Fog::Compute::Rackspace::NotFound.new
           end
 
         end

lib/fog/rackspace/requests/compute_v2/delete_image.rb

@@ -23,6 +23,7 @@ module Fog
       
       class Mock
         def delete_image(image_id)
+          raise Fog::Compute::RackspaceV2::NotFound.new if image_id == Fog::Rackspace::MockData::NOT_FOUND_ID
           response = Excon::Response.new
           response.status = 202
           response.body = ""