Add "Debug" button to inspect the response of a failed AJAX request

Related to #118

js/lib/components/RequestErrorModal.js

@@ -0,0 +1,25 @@
+import Modal from 'flarum/components/Modal';
+
+export default class RequestErrorModal extends Modal {
+  className() {
+    return 'RequestErrorModal Modal--large';
+  }
+
+  title() {
+    return this.props.error.message;
+  }
+
+  content() {
+    let responseText;
+
+    try {
+      responseText = JSON.stringify(JSON.parse(this.props.error.responseText), null, 2);
+    } catch (e) {
+      responseText = this.props.error.responseText;
+    }
+
+    return <div className="Modal-body">
+      <pre>{responseText}</pre>
+    </div>;
+  }
+}