allow error handlers in ajax requests

[snapwich]

Type of change

• Feature

Description of change

Allow error handlers for ajax requests for better error handling.

In addition to allowing the regular callback to default as the success handler:

ajax(url, function(responesText, xhr) {
  console.log("succeeded with", responseText);
});

the following is now accepted as well

ajax(url, {
  success: function(responseText, xhr) {
    console.log("succeeded with", responseText);
  },
  error: function(statusCode, xhr) {
    console.log("failed with status code", status);
  }
}