Suppress exceptions in some cases when working with variants

[mathbagu]

• Do you want to request a feature or report a bug?
  Bug fix
  Supress Exceptions in some cases when working with variants [ allowVariantMultiThreadAccess(true) ] powsybl-core#867

• What is the current behavior?
  The implementation of the VariantManager is not exactly the same as in Java

• What is the expected behavior?
  The implementation of the VariantManager should be exactly the same

• What is the motivation / use case for changing the behavior?
  Have both Java and C++ API as close as possible

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)

[mathbagu]

This issue is not necessary while the ThreadLocalMultiVariantContext is not implemented