For example, if you do:
```
./lightningd/lightningd --network=regtest --experimental-websocket-port=19846
```
Then you're trying to reuse the normal port as the websocket port, but this
only fails at *listen* time, when we activate connectd. Catch this too.
Fixes incorrect fatal() message, too.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>