Better feedback when variable does not exist in FMU's modelDescription file

[THEvang]

When validating a OspModelDescription file defining a variable that does not exist in the FMU's modelDescription, the output is a java exception. I think the output should be a message explicitly telling what is wrong. For instance: Variable: A used in Variable Group: Group A, is not defined in modelDescription.xml of FMU: FMU A

The exception is shown below:

Exception in thread "main" java.lang.NullPointerException at com.opensimulationplatform.core.owlbuilder.VariableGroupOwlBuilder.setVariables(VariableGroupOwlBuilder.java:185) at com.opensimulationplatform.core.owlbuilder.VariableGroupOwlBuilder.build(VariableGroupOwlBuilder.java:140) at com.opensimulationplatform.core.owlbuilder.VariableGroupOwlBuilder.setVariableGroups(VariableGroupOwlBuilder.java:173) at com.opensimulationplatform.core.owlbuilder.VariableGroupOwlBuilder.build(VariableGroupOwlBuilder.java:141) at com.opensimulationplatform.core.owlbuilder.SimulatorOwlBuilder.lambda$setVariableGroups$5(SimulatorOwlBuilder.java:74) at java.base/java.util.ArrayList.forEach(ArrayList.java:1540) at java.base/java.util.Collections$UnmodifiableCollection.forEach(Collections.java:1085) at com.opensimulationplatform.core.owlbuilder.SimulatorOwlBuilder.setVariableGroups(SimulatorOwlBuilder.java:68) at com.opensimulationplatform.core.owlbuilder.SimulatorOwlBuilder.build(SimulatorOwlBuilder.java:34) at java.base/java.util.ArrayList.forEach(ArrayList.java:1540) at com.opensimulationplatform.core.owlbuilder.SystemStructureOwlBuilder.buildSimulators(SystemStructureOwlBuilder.java:49) at com.opensimulationplatform.core.owlbuilder.SystemStructureOwlBuilder.build(SystemStructureOwlBuilder.java:27) at com.opensimulationplatform.core.validation.ValidatorContextFactory.create(ValidatorContextFactory.java:26) at com.opensimulationplatform.core.validation.Validator.validate(Validator.java:75) at com.opensimulationplatform.cli.Validator.validate(Validator.java:66) at com.opensimulationplatform.cli.Main.main(Main.java:85)