Handle error code '5' as 'NOT_FOUND' (#26)

This code is listed as NOT_FOUND in grpc: https://github.com/grpc/grpc/blob/master/doc/statuscodes.md

lib/index.js

@@ -142,7 +142,7 @@ function adapter(pubsub, opts) {
                 this.initializing = false;
                 subscription.on('message', this.onmessage.bind(this, subscription));
                 subscription.on('error', err => {
-                    if (err.code !== 404) {
+                    if (![404, 5].includes(err.code)) {
                         throw err;
                     }
                 });