Don't allow sending an empty Content-Length field.

When we got an error response from $api->download, we end up setting
$content_length to ''.  While mod_php magically corrected the empty
value we generated, HHVM/mod_proxy_fastcgi does not.

Detect the error and send a proper error response page instead.

Change-Id: I9aef108178d3a7060b1c095aae2f12dc1ccedd97
Merge "Don't allow sending an empty Content-Length field."