catch VMService connection exception (#6526)

packages/flutter_tools/lib/src/vmservice.dart

@@ -25,7 +25,12 @@ class VMService {
   /// Connect to '127.0.0.1' at [port].
   static Future<VMService> connect(int port) async {
     Uri uri = new Uri(scheme: 'ws', host: '127.0.0.1', port: port, path: 'ws');
-    WebSocket ws = await WebSocket.connect(uri.toString());
+    WebSocket ws;
+    try {
+      ws = await WebSocket.connect(uri.toString());
+    } catch (e) {
+      return new Future<VMService>.error('Failed to connect to $uri\n  $e');
+    }
     rpc.Peer peer = new rpc.Peer(new IOWebSocketChannel(ws).cast());
     peer.listen();
     Uri httpAddress = new Uri(scheme: 'http', host: '127.0.0.1', port: port);