make errors a bit more visible

glsl/raymarching.js

  document.body.innerHTML = "";
  
  var errorEl = document.createElement("pre");
  errorEl.style = "color: red; position: absolute; right: 0; bottom: 0;";
  errorEl.style = "color: red; position: absolute; right: 0; bottom: 0; background-color: black; z-index: 4";
  document.body.appendChild(errorEl);
  
  function displayError(e) {