remote-curl: include curl_errorstr on SSL setup failures

remote-curl: include curl_errorstr on SSL setup failures

For curl error 35 (CURLE_SSL_CONNECT_ERROR) users need the
additional text stored in CURLOPT_ERRORBUFFER to debug why
the connection did not start. This is curl_errorstr inside
of http.c, so include that in the message if it is non-empty.

Sometimes HTTP response codes aren't yet available, such as
when the SSL setup fails. Don't include HTTP 0 in the message.

Signed-off-by: Shawn Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>