Reject promises for in-progress operations on an MLTensor if it is destroyed

[a-sully]

This PR handles the following case:

const promise = context.readTensor(tensor);
// Destroy `tensor` before awaiting `promise`
tensor.destroy();  // This should reject

The current spec does not reject promise in this example. This PR updates the spec to match the Chromium implementation and what I assume to be the desired behavior (please speak up otherwise!)

Note that Chromium implements a similar pattern in other cases where the MLContext may be destroyed (either intentionally or unexpectedly), but specification of that work should follow #744

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[inexorabletash]

LGTM with one nitpick

[huningxin]

LGTM!