Add a customizable global error handler

[ThierryBerger]

This allows user to react to an engine error, be it panic or try to fix the problem.

Took inspiration from https://github.com/bevyengine/bevy/blob/main/crates/bevy_ecs/src/error/handler.rs.

Current Status

Proof of concept is here, I'd like to add an example or test where user code is fixing an issue (removes a rigidbody ?)

More errors should probably be added in there.

Should this be behind a feature flag ?

no-std should be accounted for: be it allow it (custom oncelock), or not supporting it.

[Ralith]

I would much rather see these errors propagated upwards where they occur. We don't need to replicate 1980s style error handling.