Add Content-Length header to thumb.php error responses

Add Content-Length header to thumb.php error responses

Without the Content-Length header, the response is sent with Transfer-Encoding:
Chunked, which is somehow mangled by mod_fastcgi. Varnish then claims the
response is malformed and declines to process it, sending the client a 503
instead.

Change-Id: I977387bf3e039926df58e3e5a14d0d0bd55201c4