message: Remove low value Filesystem::exists checks
  The `Filesystem::open` method will already throw an exception if a file
  doesn't exist. It may also produce a better exception message, since it
  lists the filename that could not be found. It's probably better to use
  the standardized `Filesystem::open` check, rather than custom
  `Filesystem::exists` checks.
diff: @@ -97,11 +97,6 @@ Tile& TileMap::getTile(NAS2D::Point<int> position, int level)
  */
  void TileMap::buildTerrainMap(const std::string& path)
  {
  - if (!Utility<Filesystem>::get().exists(path + MAP_TERRAIN_EXTENSION))
  - {
  - throw std::runtime_error("Given map file does not exist.");
  - }
  -
  const Image heightmap(path + MAP_TERRAIN_EXTENSION);
  const auto levelCount = static_cast<std::size_t>(mMaxDepth) + 1;
  @@ -210,12 +205,6 @@ NAS2D::Point<int> TileMap::findSurroundingMineLocation(NAS2D::Point<int> centerP
  */
  void TileMap::buildMouseMap()
  {
  - // Sanity checks
  - if (!Utility<Filesystem>::get().exists("ui/mouse_map.png"))
  - {
  - throw std::runtime_error("Unable to find the mouse map file.");
  - }
  -
  const Image mousemap("ui/mouse_map.png");
  // More sanity checks (mousemap should match dimensions of tile)
  