Update the ecs error handler to allow being updated more than once

[Brezak]

Objective

The application error handler can be set only once.

Solution

Use an atomic pointer to store the error handler pointer and allow setting it at any time.

Testing

Added 2 new tests.

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Showcase

The Bevy ecs error handling system has been changed to allow updating the error handler.

fn error_handler(error: BevyError, context: ErrorContext) {
   // ... do stuff.
}

// before
GLOBAL_ERROR_HANDLER.set(error_handler);

// after
set_error_handler(error_handler);
// do it twice, why not.
set_error_handler(error_handler);

[alice-i-cecile]

How do you feel about this versus #18810? I would also like to see some relative benchmarks of these approaches: I very much want to just remove the feature flag.

[Brezak]

How do you feel about this versus #18810? I would also like to see some relative benchmarks of these approaches: I very much want to just remove the feature flag.

I prefer their solution. Though I'd like to eventually make the error handler a Box<dyn Fn(...)>.

What would you like me to benchmark? Error handling is usually the cold path so I don't actually know where we'd see slowdowns.

[alice-i-cecile]

The initial impl shows huge slowdowns on the command benchmarks :( That said, I also prefer the linked solution, so I'm going to close this out and concentrate our efforts there.