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/Modal.js

@@ -109,27 +109,28 @@ export default class Modal extends Component {
   }
 
   /**
-   * Show an alert describing errors returned from the API, and give focus to
+   * Stop loading.
+   */
+  loaded() {
+    this.loading = false;
+    m.redraw();
+  }
+
+  /**
+   * Show an alert describing an error returned from the API, and give focus to
    * the first relevant field.
    *
-   * @param {Object} response
+   * @param {RequestError} error
    */
-  handleErrors(response) {
-    const errors = response && response.errors;
-
-    if (errors) {
-      this.alert = new Alert({
-        type: 'error',
-        children: errors.map((error, k) => [error.detail, k < errors.length - 1 ? m('br') : ''])
-      });
-    }
+  onerror(error) {
+    this.alert = error.alert;
 
     m.redraw();
 
-    if (errors) {
-      this.$('form [name=' + errors[0].source.pointer.replace('/data/attributes/', '') + ']').select();
+    if (error.status === 422 && error.response.errors) {
+      this.$('form [name=' + error.response.errors[0].source.pointer.replace('/data/attributes/', '') + ']').select();
     } else {
-      this.$('form :input:first').select();
+      this.onready();
     }
   }
 }

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;