Improve error wording when packager dies and simulator was on debug mode

Reviewed By: jingc

Differential Revision: D2835719

fb-gh-sync-id: 694aac79cd9a76cfac32f7808c3f1bbb85d6bfb0

Libraries/WebSocket/RCTWebSocketExecutor.m

@@ -140,7 +140,7 @@ - (void)sendMessage:(NSDictionary<NSString *, id> *)message waitForReply:(RCTWSM
   dispatch_async(_jsQueue, ^{
     if (!self.valid) {
       NSError *error = [NSError errorWithDomain:@"WS" code:1 userInfo:@{
-        NSLocalizedDescriptionKey: @"socket closed"
+        NSLocalizedDescriptionKey: @"Runtime is not ready for debugging. Make sure Packager server is running."
       }];
       callback(error, nil);
       return;