Is your feature request related to a problem? Please describe.
Gecko and Servo need to be able to tell if something is in an "error" state. wgpu-core doesn't have this ability today, and the suggested workaround (to have "baddie list", described in #638 (comment)) involves a lot of work duplicated on both Servo and Gecko.

Describe the solution you'd like
Throw out vec_map entirely. Instead, have a storage to be basically Vec<Element<T>>, where:

enum Element<T> {
  Vacant,
  Occupied(T, Epoch),
  Error,
}

Describe alternatives you've considered
We considered the baddie list on client side. We also considered storing (T, Epoch, bool) per element.

Additional context
Interestingly, vec_map isn't using anything sophisticated today, it's just Vec<Option<T>> really.
Also, it's in maintenace mode:

WARNING: THIS PROJECT IS IN MAINTENANCE MODE, DUE TO INSUFFICIENT MAINTAINER RESOURCES

In the future, we can also consider building something like that based on hibitset.