Add missing rejection handling for Promises (#7008)

* Add eslint-plugin-promise to detect uncaught rejections

* Move alert generation for errors to actions/alert

* Add missing rejection handling for Promises

* Use catch() instead of onReject on then()

Then it will catches rejection from onFulfilled. This detection can be
disabled by `allowThen` option, though.

app/javascript/mastodon/actions/alerts.js

@@ -1,3 +1,10 @@
+import { defineMessages } from 'react-intl';
+
+const messages = defineMessages({
+  unexpectedTitle: { id: 'alert.unexpected.title', defaultMessage: 'Oops!' },
+  unexpectedMessage: { id: 'alert.unexpected.message', defaultMessage: 'An unexpected error occurred.' },
+});
+
 export const ALERT_SHOW    = 'ALERT_SHOW';
 export const ALERT_DISMISS = 'ALERT_DISMISS';
 export const ALERT_CLEAR   = 'ALERT_CLEAR';
@@ -22,3 +29,21 @@ export function showAlert(title, message) {
     message,
   };
 };
+
+export function showAlertForError(error) {
+  if (error.response) {
+    const { data, status, statusText } = error.response;
+
+    let message = statusText;
+    let title   = `${status}`;
+
+    if (data.error) {
+      message = data.error;
+    }
+
+    return showAlert(title, message);
+  } else {
+    console.error(error);
+    return showAlert(messages.unexpectedTitle, messages.unexpectedMessage);
+  }
+}