Type confusion in mrb_exc_set leading to memory corruption

How it was found

Similar to #181871, but the bug is more general. The E_*_ERROR macros are not constants, so the exception types can be redefined to not be exceptions:

#define E_NOTIMP_ERROR              (mrb_class_get(mrb, "NotImplementedError"))

This means that any code calling mrb_raise on an exception macro can instead get a non-exception object, leading to memory corruption and arbitrary code execution. This snippet causes a native crash in mruby-engine:

NotImplementedError = String
Module.constants # mrb_raise(mrb, E_NOTIMP_ERROR, "Module.constants not implemented");

This should be fixed by making mrb_exc_set check that it is an exception type. Attached is a patch to mruby to fix this problem.