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/admin/src/components/EditGroupModal.js

@@ -85,13 +85,12 @@ export default class EditGroupModal extends Modal {
       namePlural: this.namePlural(),
       color: this.color(),
       icon: this.icon()
-    }).then(
-      () => this.hide(),
-      (response) => {
+    }, {errorHandler: this.onerror.bind(this)})
+      .then(this.hide.bind(this))
+      .catch(() => {
         this.loading = false;
-        this.handleErrors(response);
-      }
-    );
+        m.redraw();
+      });
   }
 
   deleteGroup() {