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/utils/RequestError.js

@@ -1,6 +1,15 @@
 export default class RequestError {
-  constructor(message, responseText) {
-    this.message = message;
+  constructor(status, responseText, xhr) {
+    this.status = status;
     this.responseText = responseText;
+    this.xhr = xhr;
+
+    try {
+      this.response = JSON.parse(responseText);
+    } catch (e) {
+      this.response = null;
+    }
+
+    this.alert = null;
   }
 }