http: use normalize_curl_result() instead of manual conversion

When we switched off CURLOPT_FAILONERROR in 17966c0a63 (http: avoid disconnecting on 404s for loose objects, 2016-07-11), the fetch_object() function started manually handling 404's. Since we now have normalize_curl_result() for use elsewhere, we can use it here as well, shortening the code. Note that we lose the check for http/https in the URL here. None of the other result-normalizing code paths bother with this. Looking at missing_target(), which checks specifically for an FTP-specific CURLcode and "http" code 550, it seems likely that git-over-ftp has been subtly broken since 17966c0a63. This patch does nothing to fix that, but nor should it make anything worse (in fact, it may be slightly better because we'll actually recognize an error as such, rather than assuming CURLE_OK means we actually got some data). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>