Adding error handling to code example

README.md

@@ -34,11 +34,14 @@ Your JavaScript could look like that:
 
 ```javascript
 async function translate(idPatterns) {
-    return fetch(document.location, {headers: {
+    const response = await fetch(document.location, {headers: {
         'X-Translation-Request': idPatterns,
         'Accept-Language': document.documentElement.lang
-    }})
-        .then(response => response.json());
+    }});
+    if (! response.ok) {
+        return Promise.reject('Unexpected server response');
+    }
+    return await response.json();
 }
 ```