Reduce error level of some nullptr catches in PFO/FE comparison plots
Hi
This merge request changes some of the error levels relating to the cases where Flow Elements/PFOs in the AOD have some negative energy calo clusters removed, which is now handled in a warning state and skipped. This catch was necessary since PFO/FE matching is done using said calo clusters (so if one does not exist, it is not matched etc etc). It ended up crashing some PhysVal jobs because an ERROR state is flagged and is considered FATAL when in this case it should be a skippable error.
If preferred this can be moved to DEBUG instead, but since it is somewhat a useful warning (to point out the existence of the thinning in the AOD and how it related to the cluster matching) I prefer the WARNING state.
This should rectify https://its.cern.ch/jira/browse/ATLPHYSVAL-744
Thanks
Matt A.