Use node 8 error code (#1557)

src/client/voice/opus/OpusEngineList.js

@@ -9,7 +9,7 @@ function fetch(Encoder, engineOptions) {
   try {
     return new Encoder(engineOptions);
   } catch (err) {
-    if (err.message.includes('Cannot find module')) return null;
+    if (err.code === 'MODULE_NOT_FOUND') return null;
 
     // The Opus engine exists, but another error occurred.
     throw err;