CancellationErrorContext.getWaitUntilReadyContext can throw ClassCastException

Suggested release note text:

If you call `CancellationErrorContext.getWaitUntilReadyContext()` on an error context that didn't come from a "wait until ready" request, the method is now guaranteed to return null instead of sometimes throwing a `ClassCastException`.

It's possible to construct a `CancellationErrorContext` with an `AbstractContext` that isn't a `WaitUntilReadyContext`. When this happens, `getWaitUntilReadyContext` throws ClassCastException. It should probably return null instead.