- `OP_EXCEPT` checks if `mrb->exc` is `NULL`, `MRB_TT_EXCEPTION` or
  `MRB_TT_BREAK`.
  If `mrb->exc` is `NULL`, it will be replaced with `nil`.

- If `OP_RAISE` is `nil`, it does nothing and the immediately
  following instruction is executed (like `OP_NOP`).
  Also, in case of `RBreak` object, it moves to the processing for
  `break`.
  With this change, the instruction name is changed from
  `OP_RAISE` to `OP_RAISEIF`.