TauAnalysisTools: downgrade FATAL to WARNING in BuildTruthTaus

Hello,

As discussed in ATLASG-1661, when building TruthTaus from the truth particle container in Herwig7, it seems that tau decays involving an intermediate a1 meson are buggy: the a1 doesn't decay, which creates problems in BuildTruthTaus when we navigate through the tau decay products. When an unexpected decay mode is found, we call a function that recursively prints the daughters of the tau, and at that point we encounter a second "bug/feature": some status-1 photons have a decay vertex with 2 daughters, but these daughters are not present in the truth record (there is suspicion that it may come from photon conversions in Geant4, which are thinned from the truth record). When this happens, we currently print a FATAL error message, but we don't abort the execution with a FAILURE, it's just a printout. However this causes derivation jobs to fail when parsing log files.

This MR downgrades the FATAL to a WARNING. But there seem to be an issue with Herwig7(+G4?), which will have to be followed up with experts.

Adding the urgent label as it is blocking an imminent DAOD production.

Tagging @calpigia and @akaczmar .

Cheers, Bertrand