unhelpful error from `client.run` with dead scheduler

[dchudz]

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.

[dchudz]

I'm really sorry but this was either with dask & distributed 2021.11.2, or 2022.1.1. I don't remember which. If it matters I can try to repro with latest.