Improve client XHR error handling

The default XHR error handler produce an alert which is appropriate to the response status code. It can be overridden per-request (by specifying the `errorHandler` option) so that the alert can be suppressed or displayed in a different position (e.g. inside a modal).

ref #118

js/lib/components/RequestErrorModal.js

@@ -5,10 +5,6 @@ export default class RequestErrorModal extends Modal {
     return 'RequestErrorModal Modal--large';
   }
 
-  title() {
-    return this.props.error.message;
-  }
-
   content() {
     let responseText;