Clarify an error message in swf_assets:manifests task

It's not just that none of them were 200 OK, it's that they were all 404.
In the event that something returns not-200 and not-404, we immediately
abort, so we shouldn't get to this case unless they were all 404!

lib/tasks/swf_assets.rake

@@ -89,5 +89,5 @@ def infer_manifest_url(swf_url, internet)
   end
 
   # Otherwise, there's no valid manifest URL.
-  raise "none of the common manifest URL patterns returned HTTP 200"
+  raise "all of the common manifest URL patterns returned HTTP 404"
 end