Sometimes when we crash in the GC we'd like to get some more information about what was going on the crashing thread. One example is when Generational ZGC crashes during store barrier flushing. From https://github.com/openjdk/zgc/blob/349cf9ae38664991879402a90c5e23e291f1c1c3/src/hotspot/share/gc/z/zStoreBarrierBuffer.cpp#L245

```
class ZStoreBarrierBuffer::OnError : public VMErrorCallback {
private:
  ZStoreBarrierBuffer* _buffer;

public:
  OnError(ZStoreBarrierBuffer* buffer) :
      _buffer(buffer) {}

  virtual void call(outputStream* st) {
    _buffer->on_error(st);
  }
};

void ZStoreBarrierBuffer::on_error(outputStream* st) {
  st->print_cr("ZStoreBarrierBuffer: error when flushing");
  st->print_cr(" _last_processed_color: " PTR_FORMAT, _last_processed_color);
  st->print_cr(" _last_installed_color: " PTR_FORMAT, _last_installed_color);

  for (int i = current(); i < (int)_buffer_length; ++i) {
    st->print_cr(" [%2d]: base: " PTR_FORMAT " p: " PTR_FORMAT " prev: " PTR_FORMAT,
        i,
        untype(_base_pointers[i]),
        p2i(_buffer[i]._p),
        untype(_buffer[i]._prev));
  }
}

void ZStoreBarrierBuffer::flush() {
  if (!ZBufferStoreBarriers) {
    return;
  }

  OnError on_error(this);
  VMErrorCallbackMark mark(&on_error);

  for (int i = current(); i < (int)_buffer_length; ++i) {
    const ZStoreBarrierEntry& entry = _buffer[i];
    const zaddress addr = ZBarrier::make_load_good(entry._prev);
    ZBarrier::mark_and_remember(entry._p, addr);
  }

  clear();
}
```

If we crash in ZStoreBarrierBuffer::flush, we print the information above into the hs_err file.

We've found this information to be useful and would like to upstream the infrastructure separately from the much larger Generational ZGC PR.