[bug] Skyplane gateways auto-deprovision before transfer is complete

[sarahwooders]

I've gotten SSH/HTTP connection errors in the following cases, as a result of the gateways deprovisioning (due to the auto-terminate configuration)

• During transfers for large number of files, where dispatch time will be very long so the gateways deprovision before dispatching is complete.
• During sync jobs where there is significant overlap in the files in the source and destinations, so there is a long time before any chunk is dispatched.

We should also throw a better error message (instead of HTTP/SSH connection closed) informing the users that the dataplane has been deprovisioned when this happens.