Add descriptive error message to assert in WorkerState._transition_cancelled_error

[hendrikmakait]

This PR adds a descriptive error message to the assert statement in WorkerState._transition_cancelled_error to be able to see the actual ts.previous.

For context: When scaling up tensordot_stress in coiled/benchmarks#229 I ran into the assert statement which returned a not-so-helpful

Aug 11 10:00:05 ip-10-0-3-173 cloud-init[1264]:     assert ts.previous in ("executing", "long-running")
Aug 11 10:00:05 ip-10-0-3-173 cloud-init[1264]: AssertionError

• Tests added / passed
• Passes pre-commit run --all-files

[fjetter]

Do you, by chance, have a cluster dump of the cluster where this exception was raised?

[hendrikmakait]

@fjetter: I'm trying to recreate this. Unfortunately, I tore down the cluster before stumbling across this in the logs.

[crusaderky]

ts.previous = flight. I can confidently state this without a dump or a reproducer. Discussion in #6877.