http: avoid empty error messages for some curl errors

When asked to fetch over SSL without a valid
/etc/ssl/certs/ca-certificates.crt file, "git fetch" writes

	error:  while accessing https://github.com/torvalds/linux.git/info/refs

which is a little disconcerting.  Better to fall back to
curl_easy_strerror(result) when the error string is empty, like the
curl utility does:

	error: Problem with the SSL CA cert (path? access rights?) while
	accessing https://github.com/torvalds/linux.git/info/refs

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -846,8 +846,13 @@ static int http_request(const char *url, void *result, int target, int options)
 				init_curl_http_auth(slot->curl);
 				ret = HTTP_REAUTH;
 			}
-		} else
+		} else {
+			if (!curl_errorstr[0])
+				strlcpy(curl_errorstr,
+					curl_easy_strerror(results.curl_result),
+					sizeof(curl_errorstr));
 			ret = HTTP_ERROR;
+		}
 	} else {
 		error("Unable to start HTTP request for %s", url);
 		ret = HTTP_START_FAILED;