don't throw exception on missing shader

fggl/gfx/ogl/shader.cpp

@@ -121,7 +121,11 @@ namespace fggl::gfx {
 	}
 
 	ShaderCache::ShaderPtr ShaderCache::get(const std::string &name) {
-		return m_shaders.at(name);
+		auto itr = m_shaders.find(name);
+		if ( itr != m_shaders.end() ){
+			return itr->second;
+		}
+		return nullptr;
 	}
 
 	ShaderCache::ShaderPtr ShaderCache::load(const ShaderConfig &config) {