make the output red if it's an error

go/linguaevalia.go

    #code {
      border: none;
    }
    .error { color: red; }
    </style>
  </head>
      codeEl.onkeydown = function(ev) {
        if (ev.ctrlKey && ev.keyCode == 13) {
          resultEl.textContent = "";
          sendCode(codeEl.value, function(result) {
            resultEl.textContent = result;
          sendCode(codeEl.value, function(xhr) {
            resultEl.className = xhr.status == 200 ? "success" : "error";
            resultEl.textContent = xhr.response;
          });
        }
      }
        xhr.open("POST", "/run");
        xhr.onreadystatechange = function(ev) {
          if (xhr.readyState == XMLHttpRequest.DONE) {
            cb(xhr.response);
            cb(xhr);
          }
        };
        xhr.send(code);