params.js

// I swiped this from a stack overflow question somewhere, it works well.
window.getParameterByName = function(name) {
  name = name.replace(/[\[]/, "\\\[").replace(/[\]]/, "\\\]");
  var regexS = "[\\?&]" + name + "=([^&#]*)";
  var regex = new RegExp(regexS);
  var results = regex.exec(window.location.search);
  if(results == null)
    return "";
  else
    return decodeURIComponent(results[1].replace(/\+/g, " "));
}

// Usage

if (getParameterByName("error")) {
  // display error message
}