Fix entity does not exist message on index reuse

[SpecificProtagonist]

Objective

With the track_location feature, the error message of trying to acquire an entity that was despawned pointed to the wrong line if the entity index has been reused.

Showcase

use bevy_ecs::prelude::*;

fn main() {
    let mut world = World::new();
    let e = world.spawn_empty().id();
    world.despawn(e);
    world.flush();
    let _ = world.spawn_empty();
    world.entity(e);
}

Old message:

Entity 0v1 was despawned by src/main.rs:8:19

New message:

Entity 0v1 does not exist (its index has been reused)

[chescock]

Looks good! I left some style suggestions, but nothing blocking.

The test doesn't get run by CI, right? Maybe we should create an issue to run tests with the track_location feature enabled. This probably won't be the last time we want tests for it!