MINOR: http-htx: Emit a warning if an error file runs over the buffer's reserve

MINOR: http-htx: Emit a warning if an error file runs over the buffer's reserve

If an error file is too big and, once converted in HTX, runs over the buffer
space reserved to headers rewritting, a warning is emitted. Because a new set of
rules will be added to allow headers rewritting on all responses, including
HAProxy ones, it is important to always keep this space free for error files.