async makeRequest function error-handling

[njsfield]

Great reusable implementation! As it is used in a waterfall chain, perhaps pass an Error to the callback if XMLHttpRequest doesn't exist. So instead of this;

  if (!httpRequest) {
    alert('Giving up :( Cannot create an XMLHTTP instance');
    return false;
  }

Use this

  if (!httpRequest) {
    cb('Giving up :( Cannot create an XMLHTTP instance');
  }

Otherwise the callback chain may not pick up on the 'return False' statement