dynamicproxy: Let 403 and 404 responses pass through

Lighttpd handles these already. In addition, these aren't always
emitted by the file system. Web applications can (should!) use
proper http status codes and should not get their responses
replaced by the proxy. Cf. bug 64393.

Change-Id: Ied73974ce9512d0d9a56389b3ac687ebea8bd03e
Merge "dynamicproxy: Let 403 and 404 responses pass through" into production