unhelpful error from `client.run` with dead scheduler

I have a client that was connected to a cluster (now dead).

```
In [15]: client.run(hi)
...
-> 2426 responses = await self.scheduler.broadcast(
2427 msg=dict(
2428 op="run",

AttributeError: 'NoneType' object has no attribute 'broadcast'
```

Seems like it'd be nicer to get a clear message like `Sorry, the scheduler doesn't want to talk to us anymore.`

I can try to get better repro steps if that would help you.