core: fix the error condition for Datadog report

[woshilapin]

Ooops, should be false below 400, not true.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (4105b19) 27.88% compared to head (6f8adec) 27.88%.

Files	Patch %	Lines
core/src/main/java/fr/sncf/osrd/cli/TkDataDog.kt	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##                dev    #6628   +/-   ##
=========================================
  Coverage     27.88%   27.88%           
  Complexity     2175     2175           
=========================================
  Files          1044     1044           
  Lines        127897   127897           
  Branches       2605     2605           
=========================================
+ Hits          35666    35668    +2     
+ Misses        90712    90710    -2     
  Partials       1519     1519           

Flag	Coverage Δ
core	79.36% <0.00%> (ø)
editoast	75.23% <ø> (+<0.01%)	⬆️
front	8.42% <ø> (ø)
gateway	2.50% <ø> (ø)
railjson_generator	87.25% <ø> (ø)
tests	81.95% <ø> (ø)

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