Add a class OnVMError which uses the VMErrorCallback mechanism which is an ergonomic construction for creating ad-hoc VMErrorCallback which automatically calls the provided invocable f if a VM crash occurs within its lifetime. Can be used to instrument a build for more detailed contextual information gathering. Especially useful when hunting down intermittent bugs, or issues only reproducible in environments where access to a debugger is not readily available. Example use:
  {
    // Note the lambda is invoked after an error occurs within this thread,
    // and during on_error's lifetime. If state prior to the crash is required,
    // capture a copy of it first.
    auto important_value = get_the_value();

    OnVMError on_error([&](outputStream* st) {
      // Dump the important bits.
      st->print("Prior value: ");
      important_value.print_on(st);
      st->print("During crash: ")
      get_the_value().print_on(st);
      // Dump whole the whole state.
      this->print_on(st);
    });

    // Sometimes doing a thing will crash the VM.
    do_a_thing();
  }

C++17 class template argument deduction finally makes these sort of constructions ergonomic to use without the need for auto and helper construction methods.