Improve resiliency of the Transmodel API

[hannesj]

Summary

This PR contains two one improvement:

• If an error is thrown from the parsing of the request, we generate a TripPlan, which we can show to the user together with the error. In this we include coordinates, which might be null, which will lead to an NPE. Instead pass in null all the way, and serialize it as 0 in the API, as it is a non-nullable type.
• Validation was made stricter to relaxGeneralizedCostAtDestination recently, which will throw an error if an invalid value is specified. Instead pre-validate the value and only set it if valid. Removed, as all clients are now fixed. However we should improve validation somehow already when parsing the value.

[codecov]

Codecov Report

Base: 62.14% // Head: 62.13% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (14ee947) compared to base (bfbb282).
Patch coverage: 12.50% of modified lines in pull request are covered.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #4806      +/-   ##
=============================================
- Coverage      62.14%   62.13%   -0.01%     
- Complexity     13002    13004       +2     
=============================================
  Files           1650     1650              
  Lines          66064    66076      +12     
  Branches        7184     7188       +4     
=============================================
+ Hits           41057    41058       +1     
- Misses         22667    22679      +12     
+ Partials        2340     2339       -1     

Impacted Files	Coverage Δ
...nner/routing/algorithm/mapping/TripPlanMapper.java	30.76% <0.00%> (-13.68%)	⬇️
...er/ext/transmodelapi/model/plan/PlanPlaceType.java	70.93% <20.00%> (-7.28%)	⬇️
...g/opentripplanner/api/parameter/QualifiedMode.java	65.78% <0.00%> (-2.64%)	⬇️
.../java/org/opentripplanner/astar/model/BinHeap.java	81.94% <0.00%> (+1.38%)	⬆️
...ripplanner/model/transfer/ConstrainedTransfer.java	83.33% <0.00%> (+3.33%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.