message: Convert error message to exception
diff: #include <algorithm>
  #include <iostream>
  +#include <stdexcept>
  +
  using namespace NAS2D;
  +
  UIContainer::UIContainer()
  {
  Utility<EventHandler>::get().mouseButtonDown().connect(this, &UIContainer::onMouseDown);
  @@ -30,8 +33,7 @@ void UIContainer::add(Control* control, int x, int y)
  {
  if (control == nullptr)
  {
  - std::cout << "UIContainer::addControl(): Attempting to add a NULL Control." << std::endl;
  - return;
  + throw std::runtime_error("UIContainer::addControl(): Attempting to add a NULL Control");
  }
  add(*control, {x, y});
  