+1 for purging after creation.
Varnish is configured to cache 4xx errors for a short time (5 minutes, I think), and the above patch extends this to 15 minutes. Longer is probably not needed for errors. Purging would still be great though, for responsiveness in case of the error being cached prior to creation. But, worst case scenario (such as purges being lost, which does happen), it'll self-correct soon enough.