add exception for WebGL not supported

src/constants.js

@@ -1,4 +1,8 @@
 const canvas = document.createElement('canvas')
 const gl = canvas.getContext('webgl') || canvas.getContext('experimental-webgl')
 
+if(!gl) {
+	throw new Error('Unable to initialize WebGL. Your browser may not support it.')
+}
+
 export const MAX_TEXTURE_SIZE = gl.getParameter(gl.MAX_TEXTURE_SIZE)