MEDIUM: connection: add an error code in connections

This will be needed to improve error reporting, especially for SSL.
diff --git a/src/checks.c b/src/checks.c
index a0637c2..394d29e 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1298,6 +1298,7 @@
 
 		/* prepare a new connection */
 		conn->flags = CO_FL_NONE;
+		conn->err_code = CO_ER_NONE;
 		conn->target = &s->obj_type;
 		conn_prepare(conn, &check_conn_cb, s->check.proto, s->check.xprt, s);