Report error code on follow/rejoin failure due to non-available slot

Previously, if "repmgr standby follow" or "repmgr node rejoin" failed
due to a replication slot not being available, no error code was
returned.

repmgr-action-standby.c

@@ -3142,6 +3142,8 @@ do_standby_follow_internal(PGconn *primary_conn, PGconn *follow_target_conn, t_n
 		{
 			log_error("%s", output->data);
 
+			*error_code = general_error_code;
+
 			return false;
 		}
 	}