XD-871

an exception upon stream creation with duplicate port throws a better error message

```
xd:>stream create --definition "http | log" --name http2
19:18:11,294  WARN Spring Shell client.RestTemplate:524 - POST request for "http://localhost:9393/streams" resulted in 500 (Internal Server Error); invoking error handler
Command failed org.springframework.xd.rest.client.impl.SpringXDException: Failed to bind to: 0.0.0.0/0.0.0.0:9000. Possibly the port is already in use.
```