i18n helper fetchLocale handles 404 gracefully

ui/i18n-helper.js

@@ -25,10 +25,18 @@ const getMessage = (locale, key, substitutions) => {
   return phrase
 }
 
-async function fetchLocale (localeName) {
-  const response = await fetch(`/_locales/${localeName}/messages.json`)
-  const locale = await response.json()
-  return locale
+function fetchLocale (localeName) {
+  return new Promise((resolve, reject) => {
+    return fetch(`/_locales/${localeName}/messages.json`)
+      .then(response => response.json())
+      .then(
+        locale => resolve(locale),
+        error => {
+          log.error(`failed to fetch ${localeName} locale because of ${error}`)
+          resolve({})
+        }
+      )
+  })
 }
 
 module.exports = {