core: have a better error when block has no path

[Khoyo]

Closes #5600.

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (598b2cb) 19.59% compared to head (8a04707) 19.59%.

Files	Patch %	Lines
...n/fr/sncf/osrd/sim_infra/impl/BlockInfraBuilder.kt	0.00%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #5689      +/-   ##
============================================
- Coverage     19.59%   19.59%   -0.01%     
  Complexity     2323     2323              
============================================
  Files           886      886              
  Lines        105816   105824       +8     
  Branches       2572     2572              
============================================
+ Hits          20739    20740       +1     
- Misses        83569    83576       +7     
  Partials       1508     1508              

Flag	Coverage Δ
core	78.75% <11.11%> (-0.05%)	⬇️

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