throw exception for nonexistent languages in VOA

lib/routes/voa/index.js

@@ -13,6 +13,8 @@ module.exports = async (ctx) => {
         case 'tibetan':
             baseURL = 'https://www.voatibetan.com/';
             break;
+        default:
+            throw Error('The language specified does not exist');
     }
 
     const response = await got.get(baseURL + 'api/' + ctx.params.channel || '');