fix: fall back to fullscreen if display size is negative
fix: fall back to fullscreen if display size is negative

Until now we throw exception if display size is negative. We should fall back to fullscreen mode to
avoid confusing behaviour." xibosignage#172

Stivius/xibo-linux,Stivius/xibo-linux,xibosignage/xibo-linux,xibosignage/xibo-linux