chat agent executor: catch parsing errors in tool calls

[ccurme]

                         +-----------+
                         | __start__ |
                         +-----------+
                                *
                                *
                                *
                           +-------+
                           | agent |*
                       ****+-------+ ****
                   ****          *       ****
               ****              *           ****
             **                   *              ****
+-----------------+               *                  **
| should_continue |**            *                    *
+-----------------+  ********    *                    *
           *       ***       ********                 *
            *         ****      *    ********         *
            *             **   *             ****     *
      +---------+         +-------+             +--------+
      | __end__ |         | retry |             | action |
      +---------+         +-------+             +--------+