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 ?

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Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 28.27%. Comparing base (b05d26f) to head (1848027).

Files	Patch %	Lines
editoast/src/views/v2/train_schedule.rs	0.00%	6 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #7971      +/-   ##
============================================
- Coverage     28.28%   28.27%   -0.01%     
  Complexity     2075     2075              
============================================
  Files          1273     1273              
  Lines        156082   156087       +5     
  Branches       3081     3081              
============================================
- Hits          44149    44136      -13     
- Misses       110095   110113      +18     
  Partials       1838     1838              

Flag	Coverage Δ
core	75.03% <ø> (ø)
editoast	70.96% <0.00%> (-0.06%)	⬇️
front	9.93% <ø> (ø)
gateway	2.34% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	72.93% <ø> (ø)

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[Castavo]

Would not work since 500 errors could come from core but also from diesel for example...