imap-send: return with error if curl failed

curl_append_msgs_to_imap always returned 0, whether curl failed or not.
Return a proper status so git imap-send will exit with an error code
if something wrong happened.

Signed-off-by: Nicolas Morey-Chaisemartin <nicolas@morey-chaisemartin.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

imap-send.c

@@ -1490,7 +1490,7 @@ static int curl_append_msgs_to_imap(struct imap_server_conf *server,
 	curl_easy_cleanup(curl);
 	curl_global_cleanup();
 
-	return 0;
+	return res != CURLE_OK;
 }
 #endif