bpo-38631: Replace compiler fatal errors with exceptions (GH-24369)

* Replace Py_FatalError() calls with regular SystemError exceptions.
* compiler_exit_scope() calls _PyErr_WriteUnraisableMsg() to log the
  PySequence_DelItem() failure.
* compiler_unit_check() uses _PyMem_IsPtrFreed().
* compiler_make_closure(): remove "(reftype == FREE)" comment since
  reftype can also be LOCAL or GLOBAL_EXPLICIT.