[Sensitivity] Sensi analysis should fail when loadflow fails

[sylvlecl]

• Do you want to request a feature or report a bug?

Bug

• What is the current behavior?

When the initial loadflow fails, sensitivities are still computed based on the computed state, which is not valid.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

See code here: https://github.com/powsybl/powsybl-open-loadflow/compare/main...sensi-issue-when-lf-fails

• What is the expected behavior?

The analysis should report it has failed instead of providing output values. Otherwise the user will use invalid values without noticing.