http: warn on curl_multi_add_handle failures

This will be useful for tracking down curl usage errors.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -709,6 +709,8 @@ int start_active_slot(struct active_request_slot *slot)
 
 	if (curlm_result != CURLM_OK &&
 	    curlm_result != CURLM_CALL_MULTI_PERFORM) {
+		warning("curl_multi_add_handle failed: %s",
+			curl_multi_strerror(curlm_result));
 		active_requests--;
 		slot->in_use = 0;
 		return 0;