editoast: train-simulation: return SimulationFailed on 500 from core

[Castavo]

When core.fetch returns an Error made from a 500 from core, train_simulation returns Ok(SimulationResponse::SimulationFailed) rather than an Error.

This allows to still be able to call train_schedule/simulation_summary when one of the trains leads to an error in core (an assert error for example).

I left other error status codes fail because they'll likely be transient, what do you think ?